fix: type spec of v2 group consumer handle_message

[chsukivra]

Given that brod_topic_subscriber callback is typed as below and brod_group_subscriber_worker implements brod_topic_subscriber, it will pass on brod:message_set to v2 group consumers.

-callback handle_message(brod:partition(),
                         brod:message() | brod:message_set(),
                         cb_state()) -> cb_ret().

So the brod_group_subscriber_v2 behavior should probably be typed to be either message or message_set too.