rtspsrc: Add special support for ONVIF servers (custom headers, timestamp extraction, ...)

Submitted by Nicola @drakkan

Link to original bug (#762910)

Description

Created attachment 322727
extract and apply onvif timestamp

extract absolute timestamp from onvif rtp extension and apply to rtp buffers,

onvif ntp timestamp are converted to unix timestamp, the output is something like this

/GstPipeline:pipeline0/GstFakeSink:fakesink0: last-message = chain ******* (fakesink0:sink) (4434 bytes, dts: 404561:47:11.657000000, pts: 404561:47:11.657000000, duration: none, offset: -1, offset_end: -1, flags: 00004000 tag-memory ) 0x7fa7ac0132d0
/GstPipeline:pipeline0/GstFakeSink:fakesink0: last-message = chain ******* (fakesink0:sink) (4454 bytes, dts: 404561:47:11.993000000, pts: 404561:47:11.993000000, duration: none, offset: -1, offset_end: -1, flags: 00004000 tag-memory ) 0x7fa7ac0134f0

Patch 322727, "extract and apply onvif timestamp":
0001-onvifparse-extract-and-apply-onvif-timestamp.patch