Expose metadata as dataclasses - first episode

[NicolasHug]

This PR Implements docs.google.com/document/d/1OislJC2eeaI2X38Ufy8fDRJDrEj1lcSNtZ-8L-CeQm8/edit from @scotts, with the amendments recently suggested after first review rounds.

Main changes:

• Add the dataclasses from the design doc
• Expose the _core.get_video_metadata() (still private within _core).
• Expose the public stream_metadata attribute of SimpleVideoDecoder
• Added 2 new private custom ops:
  _get_container_json_metadata(decoder)
_get_stream_json_metadata(decoder, stream_index)

Notes:

• We're keeping the existing get_json_metadata() C++ and Python functions untouched. They are 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. We left a TODO for that.

Still TODO, in following diffs:

• Expose probe_video_metadata_headers() from the proposal.
• Find more descriptive names than num_frames_computed and num_frames_retrieved. There's a TODO for that
• Decide and implement logic of the bit_rate and duration_seconds properties. There are TODOs for that.

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[facebook-github-bot]

@NicolasHug has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@NicolasHug merged this pull request in c82f142.