rtspsrc: pass seek-seqnum through the caps

The rationale for this is explained in:
gstreamer/gst-plugins-base!112
8 jobs for rtspsrc-seqnum in 19 minutes and 11 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #94583
gst indent

00:00:34

passed #94582
manifest

00:00:40

 
  Build
passed #94584
build fedora x86_64

00:03:17

 
  Test
passed #94585
check fedora

00:04:11

 
  Full Builds
passed #94586
build android arm64 api28

00:03:12

passed #94588
build cerbero cross win32

00:08:40

passed #94589
build cerbero cross win64

00:09:09

passed #94587
build cerbero fedora x86_64

00:07:58