)

* Always return a contract from the newsfeed plugin

Without a contract, dependent plugins have no way of knowing whether the
plugin is enabled or not as the contract will always be undefined.

* Export newsfeed contract types from public index

So that dependent plugins can use them.

* Declare newsfeed as an optional dependency of SIEM

We're going to use the availability of the newsfeed plugin as part of our
determination for whether or not to show the security newsfeed. If users
set `newsfeed.enabled: false`, the plugin will be unavailable and the
security feed will not be shown.

* Respect global newsfeed.enabled config in Security newsfeed

The presence of the newsfeed plugin means that newsfeed.enabled is true.
If both that and our local setting are true, we will show the Security
feed.

* Prefer object type over empty interface

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>