This commit adds support for the recently introduced partial searchable snapshot (elastic#68509) to ILM.

Searchable snapshot ILM actions may now be specified with a `storage` option, specifying either
`full_copy` or `shared_cache` (similar to the "mount" API) to mount either a full or partial
searchable snapshot:

```json
PUT _ilm/policy/my_policy
{
  "policy": {
    "phases": {
      "cold": {
        "actions": {
          "searchable_snapshot" : {
            "snapshot_repository" : "backing_repo",
            "storage": "shared_cache"
          }
        }
      }
    }
  }
}
```

Internally, If more than one searchable snapshot action is specified (for example, a full searchable
snapshot in the "cold" phase and a partial searchable snapshot in the "frozen" phase) ILM will
re-use the existing snapshot when doing the second mount since a second snapshot is not required.

Currently this is allowed for actions that use the same repository, however, multiple
`searchable_snapshot` actions for the same index that use different repositories is not allowed (the
ERROR state is entered). We plan to allow this in the future in subsequent work.

If the `storage` option is not specified in the `searchable_snapshot` action, the mount type
defaults to "shared_cache" in the frozen phase and "full_copy" in all other phases.

Relates to elastic#68605