protoc-gen-openapi: reference to imported file

Hi guys, I have a case that wants to refer to a schema from an imported file from a proto file. Since the `_ref` value on the `Reference` message is in `JSON Reference Spec`, I can't refer to the imported proto file, because the imported file not generated to the openapi schema using the `JSON reference spec`.

https://github.com/google/gnostic/blob/05a0ef713d66efb7cef8d4fd3e93b8961edf9d0b/openapiv3/OpenAPIv3.proto#L485-L487

Currently, i'm using the `protoc-gen-openapiv2` from `grpc-gateway`, they provide a JSON ref schema from the imported file by defining the package and the message name on https://github.com/grpc-ecosystem/grpc-gateway/blob/5d41954a8eb8075aef0c72842296da38a151411d/protoc-gen-openapiv2/options/openapiv2.proto#L428-L429.

Is possible / any workaround to refer to the imported proto file when generating the schema from proto fiile? I hope the `protoc-gen-openapi` from gnostic also can refer to the imported profo file.

Thank you.