rtp: Add linear audio (L8, L16, L24) RTP payloaders / depayloaders

Depayloaders even set durations on the output buffer (unlike legacy C one).

L16 depayloader handles pt 10/11 correctly if no input clock-rate or channel number is set (unlike legacy one).

Channel reordering implementation should be more efficient than legacy C one.

Also added an L20 depayloader, because we can. (Removed that again for now, because the packing/unpacking gets a bit ugly and I'll handle that separately if at all)

Pipelines

L8 Sender (6ch)

gst-launch-1.0 audiotestsrc samplesperbuffer=480 ! audio/x-raw,channels=6 ! rtpL8pay2 ! udpsink host=127.0.0.1

L8 Receiver (6ch)

GST_DEBUG=*depay:6 gst-launch-1.0 udpsrc address=127.0.0.1 caps=application/x-rtp,encoding-name=L8,media=audio,clock-rate=48000,channels=6,channel-order=DV.LRLsRsCS ! rtpL8depay2 ! fakesink silent=false