Add timebase extension to interpret timestamp units

[mzanaty]

The Timestamp header is currently defined as microseconds since the Unix epoch. A more flexible approach would be arbitrary units that are interpreted with a Timebase header. Timebase can be set to 1_000_000 (1 million) to interpret Timestamp in microseconds.

[tobbee]

This would be good. I prefer a timescale that has integer numbers for all time stamps. For example, a time scale like 48000 for 48kHz audio, but 60000 for video with 60000/1001fps. This also makes it easier to translate to and from CMAF files which typically use such time scales,

What about the zero anchor point of the timescale? Does it need to be UNIX epoch, or can it be flexible?