Allow providing version_code to google play download

[robotiv]

Hi,

Great project! I noticed that it is not possible to specify the version_code of an APK to download from Google Play but the underlying Rust EFF library used has a 'version_code' field that is not populated.

https://github.com/EFForg/apkeep/blob/master/src/google_play.rs#L88

https://github.com/EFForg/rs-google-play/blob/master/gpapi/src/lib.rs#L257

Exposing this or mapping the existing version parameter in apkeep for google play would allow someone to download a specific build of an APK which would be very useful.

I am not proficient in Rust otherwise would try,

Cheers!