Bluetooth: CAP: Add bt_cap_unicast_group_get_info

Add bt_cap_unicast_group_get_info to get information about
a CAP unicast group. For now this just contain the pointer
to the BAP unicast group, which then can be used with
bt_bap_unicast_group_get_info to get information specific
to BAP. The bt_cap_unicast_group_info struct can later be
expanded with any additional information CAP may add.

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