Bluetooth: BAP: Refactor bt_bap_base

This removes the fixed size bt_bap_base, which provides
2 improvements:
1) The RAM usage of the broadcast sink has been reduced.
   For the Broadcast Sink sample it is a reduction of 120
   octets, but with much better scaling for supporting
   more or larger BASEs.
2) The functions to parse BASEs now support arbitrary sized
   BASEs, where they were previously restricted by our
   local Kconfig options. This allow us to parse any BASE
   from a remote device, without encounting memory issues.
   We are still memory restricted on the devices we
   actually want to sync to.

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