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

• I have searched the 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 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
• Support MainClass in protobuf-gradle-plugin