Support Multiple Group GUIDs in Get Messages API

Currently, the GET /groups/{guid}/messages API supports fetching messages for only a single group GUID per request. Customers who need messages from multiple groups must make separate API calls for each GUID.

A feature enhancement is requested to allow multiple group GUIDs to be passed in a single request (e.g., 10–15 GUIDs) and return the messages for all those groups in one response.