bluez5: Manage BAP linked transports

Multiple transport from the same device may share the same stream (CIS)
and group (CIG) but for different direction, e.g. a speaker and a
microphone. In this case they are linked.
In this case:
- On acquire, if another transport has already been acquired, the new
  transport should not call Acquire or TryAcquire but re-use values from
  the previously acquired transport,
- on release, the closing of transport fd and call to Release should be
  done only for the last transport.

spa/plugins/bluez5/defs.h

@@ -603,6 +603,7 @@ struct spa_bt_transport {
 	int configuration_len;
 	char *endpoint_path;
 	bool bap_initiator;
+	struct spa_list bap_transport_linked;
 
 	uint32_t n_channels;
 	uint32_t channels[64];