This refactors how the BAP broadcast source handles the
extended and periodic advertising.
First it removes the start and stop of the extended
advertising, and instead expects the application
(or upper layers) to do this.
Second it exposes API functions to get the
necessary advertising data from BAP (service data and
the BASE), which the upper layers will then also
be responsible for setting and updating.
Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>