Backend: Build group broadcast messages feature

Based on the implementation proposal specified on &8443 (comment 1038649507)

TODO:

Make sure the existing instance level broadcast messages and the new Group level broadcast messages do not interfere with each other, ie, apply scopes to them as necessary. Build the backend components required to support Group level broadcast messages.

Availability and Testing

• Adding coverage at the unit and integration test layer should suffice for these changes. E2E tests will be added once frontend portion for this feature is complete.