Add Incremental Snapshot support to RPC (#19559)

#### 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

client/src/rpc_response.rs

@@ -435,3 +435,9 @@ impl From<ConfirmedTransactionStatusWithSignature> for RpcConfirmedTransactionSt
         }
     }
 }
+
+#[derive(Serialize, Deserialize, Clone, Copy, Debug, PartialEq)]
+pub struct RpcSnapshotSlotInfo {
+    pub full: Slot,
+    pub incremental: Option<Slot>,
+}