如何在* .proto文件中定义一个空接口？

I need such a structure in the Go:

type SomeStruct struct{
    Foo bool
    Bar interface{}
}

How to define an empty interface in *.proto file? If this is not possible then how to solve the problem with the description of a field of variable type *.proto file?

Protobuf does not support generic types. Because I know all the possible types I can use Oneof : Oneof