Use the R8 lib from Google

[martinbonnin]

Instead of using my fork use the version published by Google. Also allow to change the r8 version used

// Add the r8 repo
repositories {
    maven("https://storage.googleapis.com/r8-releases/raw/") 
}

// Configure gr8
gr8 {
  val shadowJar = create("default") {
    // Use a version name
    r8Version("8.8.19")
    // or a git sha1 directly
    r8Version("887704078a06fc0090e7772c921a30602bf1a49f")
  }
}