[3.0 comipler] Support compiler with protobuf-gradle-plugin

- [x] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.

## Describe the proposal
Dubbo has a maven compiler plugin for generating protos, but the [protobuf-gradle-plugin](https://github.com/google/protobuf-gradle-plugin) does not support specifing `MainClass` . So Dubbo's compiler can not work with gradle. Hoping anyone can help to fix it.

Here are some candidate solutions
- Publish multiple maven plugins with determined `MainClass`
- Write a build script template for generating Dubbo proto files
- Write a plugin like [akka](https://plugins.gradle.org/plugin/com.lightbend.akka.grpc.gradle)
- Support `MainClass` in `protobuf-gradle-plugin`