Allow timestamps embedded in `init.mp4` and `.mpd` files to be supplied via the CLI

[aradalvand]

In automated testing contexts where Shaka Packager's output is being snapshot-tested against verified, previously generated output (.mp4, .m3u8, .mpd files, etc.), the fact that Shaka Packager injects the current time into the init.mp4 files, as well as DASH manifest files (and possibly other places as well, these are the two I personally observed), introduces non-determinism, which breaks such tests.

Consider adding a command-line option like --current_timestamp or something similar that makes it possible for the user to specify the timestamp they intend to be embedded into such files explicitly, enabling deterministic output. This could also be useful in other scenarios where the user wants to, for whatever reason, specify the timestamp themself instead of deferring to and relying on the host machine's datetime settings.