#### Problem
There's no way to get incremental snapshot information from RPC.
#### Summary of Changes
- Add new RPC method, `getHighestSnapshotSlot` that returns a `SnapshotSlotInfo`, which contains both the highest full snapshot slot, and the highest incremental snapshot slot _based on_ the full snapshot.
- Deprecate old RPC method, `getSnapshotSlot`
- Update API docs
Fixes#19579