Bluetooth: Audio: Shell: Make broadcast source generic

Implement profile-agnostic broadcast source and broadcast
stream structs, similar to the unicast stream struct.

The purpose of these are to be able to use the same structs
for multiple profiles such as BAP, CAP and TMAP.

Since only the BAP shell implements broadcast support at this
moment, the changes are minimal, but makes it easier to implement
broadcast support for other profile shells.

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