basepayload: accept seek seqnum through the caps

This is useful when used in rtspsrc: by design, it was decided that the segment event flowing through the rtpbin wrapped by rtspsrc did not have to match the seek that had gone upstream. For example the rate is already forwarded to the depayloader through "play-speed" in the caps.

As we now want validate to check whether the segment events received by sinks have identical seqnums, that match the seqnum of the seek, we have to use a similar approach to provide the depayloader with the correct seqnum.