Manage snapshot cluster privilege

[albertzaharovits]

Drawing inspiration from https://www.elastic.co/guide/en/elasticsearch/reference/current/slm-and-security.html it appears that we're lacking a kind of manage_snapshot cluster privilege.
We currently have the monitor_snapshot and create_snapshot privileges and I think it would be sensible to also add a new manage_snapshot one.

• monitor_snapshot permits listing the repositories, not only the snapshots inside the repositories, in addition to the obvious snapshot status check.
• create_snapshot extends monitor_snapshot, to also grant permission to create snapshots to existing repositories
• manage_snapshots , I think, should extend create_snapshot to permit deleting snapshots and deleting repositories.
  Importantly, I don't think it should grant permissions to create repositories, because that would grant access to transfer data to any accessible location of the user's choice (IMO manage is a fitting privilege for that).

[elasticmachine]

Pinging @elastic/es-security (Team:Security)