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_request.rs

@@ -79,6 +79,11 @@ pub enum RpcRequest {
     )]
     GetRecentBlockhash,
     GetRecentPerformanceSamples,
+    GetHighestSnapshotSlot,
+    #[deprecated(
+        since = "1.8.0",
+        note = "Please use RpcRequest::GetHighestSnapshotSlot instead"
+    )]
     GetSnapshotSlot,
     GetSignaturesForAddress,
     GetSignatureStatuses,
@@ -151,6 +156,7 @@ impl fmt::Display for RpcRequest {
             RpcRequest::GetProgramAccounts => "getProgramAccounts",
             RpcRequest::GetRecentBlockhash => "getRecentBlockhash",
             RpcRequest::GetRecentPerformanceSamples => "getRecentPerformanceSamples",
+            RpcRequest::GetHighestSnapshotSlot => "getHighestSnapshotSlot",
             RpcRequest::GetSnapshotSlot => "getSnapshotSlot",
             RpcRequest::GetSignaturesForAddress => "getSignaturesForAddress",
             RpcRequest::GetSignatureStatuses => "getSignatureStatuses",