Bluetooth: audio: tbs_client: Minor Kconfig refactor

This intoduces dedicated Kconfig option to enable TBS client,
that makes the configuration more intuitive.
While configuring GTBS only, the user does not have to explicitly set
the BT_TBS_CLIENT_MAX_TBS_INSTANCES to 0 to disable the TBS client
functionality.
This adds also `tbs_only_client` test case to test the TBS only build
option.

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