requesthandler: Add NotReady request status

During scene collection change and OBS exit, performing requests
constitutes undefined behavior. Previously, we attempted to help this
by providing the `SceneCollectionChanging` event for any clients to
pause requests client-side, but this method has proven to not be
adequate to fix the issue. As such, this allows us to tell clients
explicitly that a request cannot be fulfilled reliably, and they
may decide whether or not to retry.

src/requesthandler/types/RequestStatus.h

@@ -103,6 +103,19 @@ namespace RequestStatus {
 		* @api enums
 		*/
 		UnsupportedRequestBatchExecutionType = 206,
+		/**
+		* The server is not ready to handle the request.
+		*
+		* Note: This usually occurs during OBS scene collection change or exit. Requests may be tried again after a delay if this code is given.
+		*
+		* @enumIdentifier NotReady
+		* @enumValue 207
+		* @enumType RequestStatus
+		* @rpcVersion -1
+		* @initialVersion 5.3.0
+		* @api enums
+		*/
+		NotReady = 207,
 
 		/**
 		* A required request field is missing.