Add Convention plugins

[krizzu]

Hey,

With the project growing, a unified build setup would be priceless. I propose to create convention plugins, so that all sub-modules (current and future) could benefit from it.

We could differentiate two plugins, one for code style (applying and configuring detect/spotless) and a build configuration (configuring compose/android min/compile version), but it's up for discussion.

I'm happy to start working on that.

[evant]

#103