[FEATURE] Low Latency transcoding pipeline options for RTMP

[mpisat]

Is your feature request related to a problem? Please describe.
When I publish track-composite to RTMP, there is this extra 1 second (or more) latency that can be avoided.

Describe the solution you'd like
Optimize buffers in gstreamer pipeline? But most importantly, -tune zerolatency preset for x264enc which makes all the difference

Describe alternatives you've considered
You could leave everything as is (HLS, mp4 recording etc) and just modify live video (RTMP, SRT maybe?)

Additional context
-tune zerolatency ensures low latency is preferred over encoding quality for x264, and vaapi (QSV) and nvenc also has similar options.

main...mpisat:egress:main
I reduced buffers agressively and added VAAPI support but I'm not planning to send a PR with this code as it is more of a draft and not handling these options exclusively for RTMP (and applies to all, which is not what I suggested above)