media: manager: support multiple codecs SDP wise

static capabilities are now supporting multiple codecs and generate correct SDP
messages. This only relates to SDP offers. When we are answering an offer we
must take the offer from the other party into account when creating the answer.

Hence why we differentiate between "static" and "dynamic" (not implemented yet)
capability strings.

plugins/sip/gst-rfc3551.h

@@ -45,3 +45,4 @@ typedef struct {
 
 MediaCodecInfo*  media_codec_by_name (const char *name);
 gchar*           media_codec_get_gst_capabilities (MediaCodecInfo *codec);
+GList*           media_codecs_get_candidates ();