rtp: In payloaders map the RTP marker flag to the corresponding buffer flag

rtp: In payloaders map the RTP marker flag to the corresponding buffer flag

This allows downstream of a payloader to know the RTP header's marker
flag without first having to map the buffer and parse the RTP header.

Especially inside RTP header extension implementations this can be
useful to decide which packet corresponds to e.g. the last packet of a
video frame.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1776>