This PR adds the first part of the Prevalidate Node Removal API. This
API allows checking whether attempting to remove some node(s) from the
cluster is likely to succeed or not. This check is useful when a node
needs to be removed from a RED cluster, without risking loosing the last
copy of some RED shards.
In this PR, we only check whether a RED index is a Searchable Snapshot
index or not, in which case the removal of any node is safe as the RED
index is backed by a snapshot.
Relates #87776
327f50ba46