Enable custom stream updates in mpas_init via callback config

[amstokely]

This PR adds an optional argument to mpas_init for supplying a user-defined callback that executes after stream manager initialization. The user provides an instance of a derived type that extends the abstract base type stream_update_callback_type. To satisfy the interface, the derived type must implement a procedure with the signature:

subroutine stream_update_callback(self, manager, ierr)
   class(stream_update_callback_type), intent(inout) :: self
   type(MPAS_streamManager_type), intent(inout) :: manager
   integer, optional, intent(out) :: ierr
end subroutine stream_update_callback

This mechanism allows users to inject custom stream modification or update logic during initialization without altering the core stream-processing code. These are the MPAS-specific changes needed to resolve issue #1373.