rtph264pay: Support STAP-A bundling

Add a new property "do-aggregate"* to the H.264 RTP payloader which
enables STAP-A aggregation as per [RFC-6184][1]. With aggregation enabled,
packets are bundled instead of sent immediately, up until the MTU size.
Bundles also end at access unit boundaries or when packets have to be
fragmented.

*: The property-name is kept generic since it might apply more widely,
   e.g. STAP-B or MTAP.
[1]: https://tools.ietf.org/html/rfc6184#section-5.7

Closes gstreamer/gst-plugins-good#434
8 jobs for stap-a in 22 minutes and 59 seconds (queued for 98 minutes and 16 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #76624
gst indent

00:00:34

passed #76623
manifest

00:00:35

 
  Build
passed #76625
build fedora x86_64

00:04:21

 
  Test
failed #76626
check fedora

00:17:28

 
  Full Builds
skipped #76627
build android arm64 api28
skipped #76629
build cerbero cross win32
skipped #76630
build cerbero cross win64
skipped #76628
build cerbero fedora x86_64
 
Name Stage Failure
failed
check fedora Test
           Total: 2906
Uploading artifacts...
gst-build/build/meson-logs/: found 2 matching files
validate-output/logs: found 3319 matching files
Uploading artifacts to coordinator... ok id=76626 responseStatus=201 Created token=1cq2xqWA
Uploading artifacts...
validate-output/logs/*.xml: found 1 matching files
Uploading artifacts to coordinator... ok id=76626 responseStatus=201 Created token=1cq2xqWA
ERROR: Job failed: exit code 1