sip: media: rework codec negotiation

introduce `calls_sip_media_manager_get_capabilities ()` which takes
a GList of MediaCodecInfo's as input to generate a SDP message.
If using in an SDP answer we simply feed it a list of the common codecs
as gathered from the SDP offer.

plugins/sip/gst-rfc3551.h

@@ -44,5 +44,6 @@ typedef struct {
 
 
 MediaCodecInfo*  media_codec_by_name (const char *name);
+MediaCodecInfo*  media_codec_by_payload_id (uint payload_id);
 gchar*           media_codec_get_gst_capabilities (MediaCodecInfo *codec);
 GList*           media_codecs_get_candidates ();