Kibana now uses the tasks API to manage automatic reindexing of the
.kibana index during upgrades.

The implementation of the tasks API requires that
1. the user executing the task can create & write to the ".tasks" index
2. the user checking on the status of the task can read (Get) the
   relevant document from the ".tasks" index

Kibana now uses the tasks API to manage automatic reindexing of the
.kibana index during upgrades.

The implementation of the tasks API requires that
1. the user executing the task can create & write to the ".tasks" index
2. the user checking on the status of the task can read (Get) the
   relevant document from the ".tasks" index

Right now using the `GET /_tasks/<taskid>` API and causing a task to opt
in to saving its result after being completed requires permissions on
the `.tasks` index. When we built this we thought that that was fine,
but we've since moved towards not leaking details like "persisting task
results after the task is completed is done by saving them into an index
named `.tasks`." A more modern way of doing this would be to save the
tasks into the index "under the hood" and to have APIs to manage the
saved tasks. This is the first step down that road: it drops the
requirement to have permissions to interact with the `.tasks` index when
fetching task statuses and when persisting statuses beyond the lifetime
of the task.

In particular, this moves the concept of the "origin" of an action into
a more prominent place in the Elasticsearch server. The origin of an
action is ignored by the server, but the security plugin uses the origin
to make requests on behalf of a user in such a way that the user need
not have permissions to perform these actions. It *can* be made to be
fairly precise. More specifically, we can create an internal user just
for the tasks API that just has permission to interact with the `.tasks`
index. This change doesn't do that, instead, it uses the ubiquitus
"xpack" user which has most permissions because it is simpler. Adding
the tasks user is something I'd like to get to in a follow up change.

Instead, the majority of this change is about moving the "origin"
concept from the security portion of x-pack into the server. This should
allow any code to use the origin. To keep the change managable I've also
opted to deprecate rather than remove the "origin" helpers in the
security code. Removing them is almost entirely mechanical and I'd like
to that in a follow up as well.

Relates to elastic#35573

Right now using the `GET /_tasks/<taskid>` API and causing a task to opt
in to saving its result after being completed requires permissions on
the `.tasks` index. When we built this we thought that that was fine,
but we've since moved towards not leaking details like "persisting task
results after the task is completed is done by saving them into an index
named `.tasks`." A more modern way of doing this would be to save the
tasks into the index "under the hood" and to have APIs to manage the
saved tasks. This is the first step down that road: it drops the
requirement to have permissions to interact with the `.tasks` index when
fetching task statuses and when persisting statuses beyond the lifetime
of the task.

In particular, this moves the concept of the "origin" of an action into
a more prominent place in the Elasticsearch server. The origin of an
action is ignored by the server, but the security plugin uses the origin
to make requests on behalf of a user in such a way that the user need
not have permissions to perform these actions. It *can* be made to be
fairly precise. More specifically, we can create an internal user just
for the tasks API that just has permission to interact with the `.tasks`
index. This change doesn't do that, instead, it uses the ubiquitus
"xpack" user which has most permissions because it is simpler. Adding
the tasks user is something I'd like to get to in a follow up change.

Instead, the majority of this change is about moving the "origin"
concept from the security portion of x-pack into the server. This should
allow any code to use the origin. To keep the change managable I've also
opted to deprecate rather than remove the "origin" helpers in the
security code. Removing them is almost entirely mechanical and I'd like
to that in a follow up as well.

Relates to #35573

Right now using the `GET /_tasks/<taskid>` API and causing a task to opt
in to saving its result after being completed requires permissions on
the `.tasks` index. When we built this we thought that that was fine,
but we've since moved towards not leaking details like "persisting task
results after the task is completed is done by saving them into an index
named `.tasks`." A more modern way of doing this would be to save the
tasks into the index "under the hood" and to have APIs to manage the
saved tasks. This is the first step down that road: it drops the
requirement to have permissions to interact with the `.tasks` index when
fetching task statuses and when persisting statuses beyond the lifetime
of the task.

In particular, this moves the concept of the "origin" of an action into
a more prominent place in the Elasticsearch server. The origin of an
action is ignored by the server, but the security plugin uses the origin
to make requests on behalf of a user in such a way that the user need
not have permissions to perform these actions. It *can* be made to be
fairly precise. More specifically, we can create an internal user just
for the tasks API that just has permission to interact with the `.tasks`
index. This change doesn't do that, instead, it uses the ubiquitus
"xpack" user which has most permissions because it is simpler. Adding
the tasks user is something I'd like to get to in a follow up change.

Instead, the majority of this change is about moving the "origin"
concept from the security portion of x-pack into the server. This should
allow any code to use the origin. To keep the change managable I've also
opted to deprecate rather than remove the "origin" helpers in the
security code. Removing them is almost entirely mechanical and I'd like
to that in a follow up as well.

Relates to #35573