Bluetooth: audio: pacs: Add bt_pacs_set_available_contexts_for_conn

This adds the function that sets the available contexts value per
connection object so that API user can set different available contexts
per connection.
The lifetime of such overriden value is the same as connection lifetime,
so on reconnection or device reboot all devices return to having the
same available contexts value again.

Fixes: #64968
Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>