Add API to mount a snapshot as a searchable index

[dakrone]

This commit adds an API for mounting a snapshot as a searchable index.

Internally, a request is made to the repository to fetch metadata information necessary from the
snapshot in question, which is then passed to the regular snapshot restore mechanism with some
specialized settings.

Assume you have an existing snapshot repository and snapshot, the API looks like:

POST /my-new-index/_snapshot/mount?wait_for_completion=true
{
  "repository": "my-s3-repo",
  "snapshot": "production-2019-12-11-aoeufao",
  "snapshot_index": "important-data",
  "index_settings": {
    "index.number_of_replicas": 2
  },
  "ignore_index_settings": ["index.refresh_interval"]
}

While the request may appear similar to a snapshot restore request, it has slight differences. The
repository, snapshot, and snapshot_index fields are required (all other fields are optional).

The response is identical to the response when restoring a snapshot. In the above example the newly
restored/mounted index will be called my-new-index.

Relates to #50999

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Snapshot/Restore)

[dakrone]

Note for reviewers: this adds the API, but I tried to make as minimal changes as possible to the rest of the searchable snapshot infrastructure. This does not remove the "searchable" repository type, and only changes a few assertions made about the repository type where necessary to have mounting work with no issues.

[tlrx]

Thanks@dakrone, and thanks for having minimized the changes. I made a first bunch of comments (I didn't look at the tests yet).

[DaveCTurner]

Thanks @dakrone, I left a few small comments too.

[DaveCTurner]

@elasticmachine please run elasticsearch-ci/bwc
@elasticmachine please run elasticsearch-ci/default-distro

[tlrx]

This looks good, thanks for picking this work David. I've left some comments, but nothing to worry about.

[DaveCTurner]

Everything addressed @tlrx, I think, this is ready for another look.

[tlrx]

LGTM - if CI is happy, I'll be happy

[DaveCTurner]

Thanks @dakrone and @tlrx 🎉