bluetooth: Introduce a can_be_supported API for A2DP codecs

This API internally checks if a requested codec can be supported on the
system. This is especially required for codecs supported via GStreamer
where the availability of a plugin decides if the said codec can be
supported.

This will be used to prevent registration of a codec which the remote
endpoint device might be able to support, but, PulseAudio can't as the
codec is not available on the system due to the absence of a plugin.
We can also prevent listing or switching to an unavailable codec.

Note that the codec negotiation happens with the bluez stack even before
a device is connected. Because of this, we need to make sure that gst_init
is called before checking for the availability of a plugin. Since
module-bluez5-device gets loaded only after a connection to the device
has been established, doing the gst_init in that or one of the bluetooth
modules is not feasible.

Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/440>

src/modules/bluetooth/bluez5-util.c

@@ -2150,6 +2150,7 @@ pa_bluetooth_discovery* pa_bluetooth_discovery_get(pa_core *c, int headset_backe
     const pa_a2dp_codec *a2dp_codec;
     char *endpoint;
 
+    pa_bluetooth_a2dp_codec_gst_init();
     y = pa_xnew0(pa_bluetooth_discovery, 1);
     PA_REFCNT_INIT(y);
     y->core = c;