Health indicator for broken repositories

[DaveCTurner]

Sometimes nodes cannot create a working snapshot Repository instance as specified by the current cluster metadata. This might be due to a missing plugin (yielding an UnknownTypeRepository instance) or some other bad config (yielding an InvalidRepository instance). This causes log messages and failures when actually trying to use the repository but may otherwise go unnoticed. I think we should have a health indicator to surface this kind of misconfiguration directly to the user.

Relates #103752.

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)