Update C++ metadata names to match python

[Dan-Flores]

This PR updates the fields of StreamMetadata and ContainerMetadata in Metadata.h to match the names of the equivalent fields in Python. This aligns the mismatched variables observed in #125.

In StreamMetadata:

• durationSeconds -> durationSecondsFromHeader
• beginStreamFromHeader -> beginStreamSecondsFromHeader
• numFrames -> numFramesFromHeader
• averageFps -> averageFpsFromHeader
• minPtsSecondsFromScan -> beginStreamSecondsFromContent
• maxPtsSecondsFromScan -> endStreamSecondsFromContent
• numFramesFromScan -> numFramesFromContent

In ContainerMetadata:

• durationSeconds -> durationSecondsFromHeader

These changes are reflected in other references to these metadata classes, in SingleStreamDecoder.h/.cpp, custom_ops.cpp, video_clip_sampler.py, and to the references in test files test_ops.py, VideoDecoderTest.cpp

[NicolasHug]

Thanks for the PR @Dan-Flores , LGTM !

Feel free to rename minPtsFromScan and maxPtsFromScan in this PR or in a follow-up, up to you.

I suspect that this PR may cause internal tests to fail, because some internal code-bases are relying on the json metadata keys that are now being renamed (example: https://fburl.com/code/0c3g3xt7).
There shouldn't be too many internal instances to fix, but to be on the safe side it might be best to import this PR as a diff before we merge it on GitHub, and pre-emptively fix issues in a separate diff to keep our diff-train healthy.

There should be an "import to fbsource" button a the bottom of this PR, next to the merge button. Happy to sync more offline on this.

[facebook-github-bot]

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

[facebook-github-bot]

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