fmp4mux, hlscmafsink: Add support for temporary fragment duration overshoot

Adding "fragment-duration-mode" and "max-fragment-duration-diff" properties to allow temporary fragment duration overshoot, so that average fragment duration can be as close as configured "fragment-duration". This can be useful in HLS production where absolute time information of a fragment may not be written in manifest, or a case where player expects sequence numbers of variants and renditions manifests are all aligned.