Add a STATE query parameter to Get snapshot API

[ElenaStoeva]

The @elastic/platform-deployment-management team is maintaining the Snapshot Restore plugin in Kibana and we would like to request adding a state query parameter to the Get snapshot API that would allow us to resolve a bug (elastic/kibana#159324) and an enhancement request (elastic/kibana#137432).

For fixing the bug (which we are currently working on), we need to fetch the last successful snapshot from a managed repository. Without any query parameter for the state of the snapshots, what we can do is fetch all snapshots and search for a successful one on the client side, but this might create performance issues for users with many snapshots. Therefore, we thought it would be more effiecient if we had a state query parameter so that we can fetch just one snapshot with a request like this:

        await clusterClient.asCurrentUser.snapshot.get({
          repository: managedRepositoryName,
          snapshot: '_all',
          ignore_unavailable: true,
          sort: 'start_time',
          order: 'desc',
          state: 'SUCCESS',
          size: 1,
        });

The second issue is another use case where we would need a state query parameter so that we can efficiently filter the list of snapshots by state without having to fetch all existing snapshots.

[elasticsearchmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[DaveCTurner]

I think this is a reasonable thing to do (and a good new-starter issue too). @original-brownbear did you mark it as team-discuss because you have doubts about that?

[original-brownbear]

@DaveCTurner no strong doubts :) just didn't want to add to this complicated API without another pair of eyes, I'm all for it and if you are too -> lets do it + it's pretty trivial to do this.

[alisonelizabeth]

Thanks for picking this up! We've had a couple support issues come through that could be resolved with this feature request.