Convert scripts in .buildscript to custom Gradle plugins

[zach-klippenstein]

Turns out it's just a lot nicer to write a custom plugin in buildSrc than have to deal with groovy in .gradle files. All you need is this:

buildSrc/src/main/java/MyPlugin.kt:

class MyPlugin : Plugin<Project> {
  override fun apply(target: Project) {
    // Go wild
  }
}

build.gradle.kts:

subprojects {
  plugins.apply(MyPlugin::class.java)
}