gst-libav: Enable `avdemux_g723_1` demuxer, and add proper CAPS for `avdec_g723_1` decoder.
Using the demuxer and the decoder in the same pipeline enables the decoding of G723.1 encoded data (audio/G723). This commit will also create a path to enabling (near-future) support for extracting G723.1 encoded audio data contained in .wav files and RTP streams.