rtp/basepayload: implement support for rtp header extensions

New signals are added for managing the internal list of rtp header
extension implementations read by a specific depayloader instance.

If the 'extmap-$NUM' field is present in the src caps, then an
extension implementation will be requested but is not required to be able
to negotiate correctly.  An extension will be requested using the
'request-extension' signal if none could be found internally.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/748>