Bluetooth: CAP: Add cap_unicast_group API

Adds a new abstract struct for unicast group that is
specific for CAP. The difference between this and the BAP
unicast group, is that the parameters are CAP streams and
thus ensuring that the streams in the group adhere to the
additional requirements that CAP has on top of BAP.

This also adds foreach functions for both CAP and BAP
to allow users to iterate on the streams in the
abstract groups.

Various samples, modules and tests have been updated
to use the CAP struct and API.

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