Expand VideoObject schema to include SeekToAction markup to handle URL and timestamp values

[lucy-kind]

Add SeekToAction type, which will be the value for potentialAction property under VideoObject.

SeekToAction will have properties for target, which takes URL as value, and startOffset-input, which is a pseudo property a la Action schema that takes Text as value to represent timestamp value to be filled in the target.

[danbri]

To contextualize this proposal more explicitly, it is a proposal from Google based on our experience with existing product features that consume /VideoObject and /Action markup for media content. As always, we won't predict or promise whether or when we'll use it (hence it is proposed for Pending) but it would fill a practical gap, and it is something we'd strongly consider consuming if standardized.

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