Bluetooth: BAP: Add bt_bap_unicast_group_get_info

Add bt_bap_unicast_group_get_info to get information
about a unicast group. In this first iteration the info
struct only contains the sink and source presentation
delay, but the info struct can easily be expanded on
later to contain more.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>