Resolve/cluster API supports no index expression

[quux00]

This enhancement provides users with the ability to query the _resolve/cluster API endpoint without specifying an index expression to match against. This allows users to quickly test what remote clusters are configured on a cluster and whether they are available for querying. Since the _resolve/cluster endpoint always attempts to make a remote connection on each call (unlike the _remote/info endpoint), this new endpoint provides a convenient and reliable way to check on the availability of remote clusters.

This new endpoint works with clusters from older versions.

The new endpoint takes no index expression:

GET _resolve/cluster

and returns the same information as before except for the "matching_indices" field. Example response:

{
  "remote1": {
    "connected": false,
    "skip_unavailable": true
  },
  "remote2": {
    "connected": true,
    "skip_unavailable": false,
    "version": {
      "number": "8.17.0",
      "build_flavor": "default",
      "minimum_wire_compatibility_version": "7.17.0",
      "minimum_index_compatibility_version": "7.0.0"
    }
  }
}

[quux00]

TODO: I need to update the resolve/cluster asciidoc end user docs, but I don't want to do that until another docs PR merges: #118993