timecodestamper: add support for closedcaption input

Some closedcaption elements like sccenc except input buffers to have timecode metas. The original use case is to serialize closed captions extracted from a video stream, in that case ccextractor copies the video time code metas to the closed caption buffers, but no such mechanism exists when creating a CC stream ex nihilo.

Remedy that by having timecodestamper accept closedcaption input caps, as long as they have a framerate.