rtpav1depay: Implement srcpad set_caps

Without this auto-pluggers such as decodebin or parsebin will be unable to process AV1 RTP payloads.

Tested with: videotestsrc num-buffers=50 ! videoconvert ! av1enc ! av1parse ! rtpav1pay ! queue ! decodebin3 ! videoconvert ! queue ! autovideosink