bluetooth: Register an endpoint according to encode or decode support

bluetooth: Register an endpoint according to encode or decode support

As we now support codecs other than SBC, we might have codec which does
not have an encode or a decode capability. Specifically, in the case of
LDAC there isn't a known decoder implementation available. For such a
case, we should not register the corresponding endpoint.

In case of LDAC, as decoding cannot be supported, we should not register
a sink endpoint or vice versa in the other scenario.

To do this, we check if encode_buffer or decode_buffer entry for a codec
has been set in pa_a2dp_codec and accordingly prevent or allow it's
registration.

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