Infer protoc version from protobuf version automatically

Because `protoc` generated code uses Protobuf internal API which doesn't have any stability guarantee, the version of `protoc` should always exactly match that of the protobuf runtime. We should make the protobuf plugin decide what version of protoc to use based the version of protobuf/protobuf-nano in the java compile dependencies, unless the user has explicitly specified a particular protoc version or binary.