feat: [VideoTranscoder] added support for specifying HEVC codec tag (#8451)

* feat: added support for specifying HEVC codec tag
feat: added support for converting the input video to a supported high dynamic range (HDR) formats
feat: added support for DROP_DUPLICATE frame rate conversion strategy to achieve the exact frame rate on the output video
feat: added support for fill_content_gaps field to allow processing inputs with significant number of missing frames
docs: improve comment formatting, add links, add references

PiperOrigin-RevId: 784316559

Source-Link: googleapis/googleapis@142ec41

Source-Link: googleapis/googleapis-gen@174c3e5
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