Refactor & cleanup widevine sig file generation process on Windows

[simonhong]

Currently, widevine sig file generation is done separately right after build is finished. https://github.com/brave/brave-browser/blob/master/lib/build.js#L113

Sig file generation script runs with system python because it was hard to resolve external dependencies(cryptography) that is not available from chromium packaged python.

If we can do this during the create installer step during the create_dist, it would be much simpler.
We can try this by installing additional dependencies to brave/vendors.