[Incremental Snapshots] Add RPC fn to query a specific snapshot's info

[brooksprumo]

At bootstrap, a node gets a list of snapshot Slot+Hashes, and then picks the highest Slot. If it needs to download the snapshot, we need to know if it's a full snapshot or an incremental snapshot. If it's an incremental snapshot, we need to download the full snapshot (if we don't have it already).

So we need a way to query the RPC node about the snapshot to figure out (1) its type, and (2) if it's an incremental snapshot then we also need the slot+hash of its corresponding full snapshot.

[brooksprumo]

Closing this issue, as a new way to handle downloading incremental snapshots at bootstrap will be used instead.

[github-actions]

This issue has been automatically locked since there has not been any activity in past 7 days after it was closed. Please open a new issue for related bugs.