[SR] Replace batched snapshot ES requests with single request

[jen-huang]

In master (v8.0.0), the /api/snapshot_restore/snapshots endpoint currently still uses multiple ES requests to retrieve information about all snapshots: LOC

This is because due to previous ES API limitations, there was no way to retrieve information about all snapshots, from all repositories. The current logic first retrieves all repositories, then retrieves all snapshots from each repository.

With new changes in ES, GET /_snapshot/_all/_all to retrieve all snapshots from all repositories is now available. For performance, we should update this endpoint handler logic to use this new ES API instead.

Note: this change should be made against master only, as the ES changes are only available for 8.0.0.

[elasticmachine]

Pinging @elastic/es-ui