pub fn extern_path(mut self, proto_path: impl AsRef<str>, rust_path: impl AsRef<str>) -> Self {
        self.extern_path.push((
            proto_path.as_ref().to_string(),
            rust_path.as_ref().to_string(),
        ));
        self
    }
Declare externally provided Protobuf package or type.

Passed directly to `prost_build::Config.extern_path`.
Note that both the Protobuf path and the rust package paths should both be fully qualified.
i.e. Protobuf paths should start with "." and rust paths should start with "::"
tonic-build/src/prost.rs