Bluetooth: Audio: Shell: Add broadcast assistant add broadcast_id

Add command to add a source by the broadcast ID. This will
initiate scanning for the broadcast ID, making it unnecessary
for the user to keep track of both the broadcast ID, the SID
and the address, and just focus on the (properly) known
broadcast ID.

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