Draft: gstrtpjitterbuffer: Support x-gst-netclock attributes

This is a custom GStreamer SDP attribute extension for implementing RFC 7273 clock sync using a GstNetTimeProvider server. This has been tested with rtspsrc.

Ideally this also needs documentation changes to document it as an SDP extension for people trying to search the internet for what it is. I'll keep this as a draft till I can get to that.

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