Bluetooth: audio: tbs_client: Separate GTBS from TBS instances

This improves the code readability and reduces flash usage a bit by
spliting up GTBS from other TBS instances in bt_tbs_server_inst
structure, so that some of the code could be conditionally compiled
out.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>