pbutils: Add function to parse RFC 6381 codecs field

This is the opposite of `gst_codec_utils_caps_get_mime_codec()`, which allows
elements to get the `GstCaps`

Part-of: <gstreamer/gstreamer!1894>
25 jobs for !1894 with rfc-6381 in 30 minutes and 56 seconds (queued for 4 seconds)
merge request