Add Snapshots endpoint for retrieving all snapshots

[cjcenizal]

Per discussion with @ywelsch, the internal requests currently only support a single repo, but could be extended to multiple repositories. This will also require adaptions to the response objects (so that it is clear from which repo the response is coming from). Note that it's technically possible for the same snapshot to belong to multiple repositories.

This is related to #15151, which added support for wildcards.

Some background behind this. The ES UI team is building a Snapshot and Restore app which surfaces a table of all snapshots, with the related repository. Currently, this requires retrieving all repositories, looping over them, and sending a request to retrieve all snapshots for each repository.

In addition to providing information about which repository a snapshot belongs to, the endpoint should also return a collection of any errors that repositories respond with, associated with the name of the repository.

[elasticmachine]

Pinging @elastic/es-distributed

[cjcenizal]

CC @jen-huang