Gradle DSL does not work with version catalogs

You cannot do the following, for example.

```kotlin
protobuf {
plugins {
id("grpc") {
setArtifact(libs.whatever)
}
}
}
```

`libs.whatever` returns a Provider of type [MinimalExternalModuleDependency](https://docs.gradle.org/current/javadoc/org/gradle/api/artifacts/MinimalExternalModuleDependency.html) but the method only accepts strings.

Would be nice to support [version catalogs](https://docs.gradle.org/current/userguide/platforms.html).