[WIP] Add get_video_metadata() and associated dataclasses

[NicolasHug]

This PR Implements https://docs.google.com/document/d/1OislJC2eeaI2X38Ufy8fDRJDrEj1lcSNtZ-8L-CeQm8/edit from @scotts.

Main changes:

• Add the dataclasses from the design doc, all untouched

• Added 2 new private custom ops:

  • _get_container_json_metadata(decoder)
  • _get_stream_json_metadata(decoder, stream_index)

  This is needed because the existing get_json_metadata() just returns the stream metadata of the best stream, but we want the VideoMetadata dataclass to provide the metadata of all existing streams.

Some thoughts / remarks:

• Some basic tests fail on FFmpeg 6 and 7. Looks like FFmpeg changed the way it computes the container metadata after FFmpeg 6. This looks eerily similar to this existing TODO which @ahmadsharif1 is investigating.
• I'm keeping the existing get_json_metadata() C++ and Python functions untouched. This is because it's currently in use internally, and it'll be easier to migrate that internal code in a separate diff.
• We are still in the business of manually creating a json string from C++. We may prefer doing things differently in the future but for now this is OK, and in any case revamping this isn't in scope for this PR.

Not done yet: probe_video_metadata_headers(). LMK if you prefer it to be done here, or in a follow-up PR.

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