This commit makes ILM aware of different parts of the node shutdown lifecycle. It consists are two
main parts, reacting to the state during execution, and signaling the status of shutdown from ILM.

Reacting to shutdown state
ILM now considers nodes that are going to be shut down when deciding which node to assign for the
shrink action. It uses the `NodeShutdownAllocationDecider` within the `SetSingleNodeAllocateStep` to
not assign shards to a node that will be removed. If an index is already past this step and waiting
for allocation, this commit adds an `isCompletable` method to the
`ClusterStateWaitUntilThresholdStep` so that an allocation that cannot happen can be rewound and
retried on another (non-shutdown) node.

Signaling shutdown status
This commit introduces the `PluginShutdownService` which deals with `ShutdownAwarePlugin` classes.
This class is used to signal shutdowns to plugins, and also to gather the status of a shutdown from
these plugins. ILM implements this `ShutdownAwarePlugin` to signal if an index is in a step that is
unsafe, such as the actual shrink step, so that shutdown will wait until after the allocation rules
have been removed by ILM.

This commit also hooks up the get shutdown API response to consider the statuses of its parts (see
`SingleNodeShutdownMetadata.Status#combine`) when creating a response.

Relates to elastic#70338