Allow control of protoc and grpc version via BOMs

Both protobuf and grpc publish BOM files that can be applied to a Gradle build to manage the dependency versions.

```groovy
dependencies {
api platform("io.grpc:grpc-bom:$grpcVersion")
api platform("com.google.protobuf:protobuf-bom:$protobufJavaVersion")
....
}
```

However it seems you have to hard code the version numbers in use via the `artifact` setting in the plugin. It would be good if there was a way to manage the versions of the plugin via the incorporated BOM file.