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!

[Hainish]

Thanks for the feedback. I'd love to know why specifying what the Google Play API calls the "version string" (e.g. 1.2.3) isn't sufficient for your use case, since this is the commonly known version designation for an app.