feat: Allow LIVE UDP WebVTT input

[Canta]

An updated version of PR #1027

That previous PR was done using 2021 code, and there were many changes in the codebase from there, so a rebase was needed and also some minor tweak here and there. But it's the same code, just reimplemented on a newer codebase.

If you want to take a look at this in action, after building shaka packager with this PR's code included, try this commands in 3 different simultaneous bash sessions:

1. Video UDP input: ffmpeg -f lavfi -re -i "testsrc=s=320x240:r=30,format=yuv420p" -c:v h264 -sc_threshold 0 -g 30 -keyint_min 30 -r 30 -a53cc 1 -b:v 150k -preset ultrafast -r 30 -f mpegts "udp://127.0.0.1:10000?pkt_size=1316"
2. WebVTT UDP input: for sec in $(seq 0 9999) ; do printf "%02d:%02d.000 --> %02d:%02d.000\ntest second ${sec}\n\n" "$(( ${sec} / 60 ))" "$(( ${sec} % 60 ))" "$(( (${sec} + 1) / 60 ))" "$(( (${sec} + 1) % 60 ))" ; sleep 1 ; done > /dev/udp/127.0.0.1/12345
3. shaka packager command line: timeout 60 path/to/build/packager/packager 'in=udp://127.0.0.1:10000?timeout=8000000,stream_selector=0,init_segment=240_init.m4s,segment_template=240_$Number%09d$.m4s,bandwidth=150000' 'in=udp://127.0.0.1:12345?timeout=8000000,stream_selector=0,input_format=webvtt,format=webvtt+mp4,init_segment=text_init.m4s,segment_template=text_$Number%09d$.m4s,language=eng,dash_roles=subtitle' --mpd_output ./manifest.mpd --segment_duration 3.2 --suggested_presentation_delay 3.2 --min_buffer_time 3.2 --minimum_update_period 3.2 --time_shift_buffer_depth 60 --preserved_segments_outside_live_window 1 --default_language=eng --dump_stream_info 2>&1

Note the added input_format=webvtt to the shaka packager command's second selector. That's new from this PR. If you don't use that, shaka's format autodetection will not detect the webvtt format from the input, as explained in #685 (comment). Try the command without it if you want to.

Fixes #685
Fixes #1017

[cosmin]

Overall lgtm but I think the documentation for stream descriptor also needs to be updated.

[joeyparrish]

Agreed.

[Canta]

@cosmin @joeyparrish Thank you both for kindly take a look at this PR.
I'm not familiar with the docs, so did some greps over the code and guess that last commit should be enough for a single selector option added. Please let me know if there's something else I should do.

[joeyparrish]

Looks good to me. If the tests pass, I'll merge this.