Enable support for `audio[video]/ogg` mime type in Gstreamer backend

[tharkum]

Some WPT tests are using audio/ogg or video/ogg with the next conbination

FAIL (audio/video + source with type)

<audio><source src="any-audio-content.oga" type="audio/ogg">
<video><source src="any-video-content.ogv" type="video/ogg">

After implementation of supported selection of media element source from source children (PR servo/servo#39717) there was found that GStreamer backend is reporting that audio/ogg or video/ogg mime type is not supported (hardcoding).

See https://html.spec.whatwg.org/multipage/media.html#loading-the-media-resource:attr-source-type

Need to figure out is it possible to enable it via registry backends/gstreamer/lib.rs

Associated Gstreamer issu was closed (merged/backported to 1.24?!) 8 months ago (gst-plugins-base/issues/520)