Checking whether rtpjitterbuffer actually timestamps the buffers according to
the RFC7273 clock definition and rtpjitterbuffer configuration required looking
at the DEBUG logs.
This commit adds an entry in the rtpjitterbuffer stats to indicate if
conditions are met for RFC7273 to be active.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7829>