Option to get widevine from lacros image

[horstle]

This adds the possibility to extract Widevine from a Lacros image. It also splits code that is specific to getting Widevine from Googles repo into repo.py.

Currently it relies on the zstd library being installed on the system. If that's not the case, it will use the old ChromeOS image method. It would be better if there was an addon for zstd decompression.