feat: removed -latest flag and instead support latest version marker, refactor

[filipowm]

Hey,

I'm dropping next one with few minor changes, mostly around making code generator a bit easier to read through and understand, but also simplify it. It was mostly around moving code away from main.go, which now is cleaner and acting only as CLI entrypoint, to dedicated files.

1. Removed -latest flag and instead you can specify latest (being a default when no specific version provided) as a version.
   From:

go:generate go run ../fields/ -version-base-dir=../fields/ -latest
go:generate go run ../fields/ -version-base-dir=../fields/ 8.6.9

To:

go:generate go run ../fields/ -version-base-dir=../fields/
go:generate go run ../fields/ -version-base-dir=../fields/ latest # equivalent of above
go:generate go run ../fields/ -version-base-dir=../fields/ 8.6.9

2. Moved code responsible for determing unifi version and download URL to version.go, together with writing version file
3. Moved code downloading deb and extracting from it files to download.go (renamed from extract.go
4. Moved all code generation to generator.go

I tested changes additionally manually on few versions of controller and everything works as before.