Bluetooth: Audio: Broadcast Assistant: Check add_source duplication

Added a check for checking duplication when adding a source in the
broadcast assistant, as per requirement in BAP 6.5.4. It is not
permitted to add a source such that a combination of broadcast_id,
adv_sid and address type would result in same triple of values.
Added a recv_state struct to cache these values when read, and moved
past_avail and srd_ids into this as well.

Signed-off-by: Fredrik Danebjer <frdn@demant.com>