Fix validation of gRPC stub with nested types

[ashashev]

Problem:

The validation fails if the request prerequisite refers to a field with a nested type or request/response classes has a nested type.

Example:

proto-file:

syntax = "proto3";

message V1 {
  message Request {
    enum Order {
      ORDER_UNKNOWN = 0;
      ORDER_ASC = 1;
      ORDER_DESC = 2;
    }
    Order order = 1;
  }

  message Response {
    enum Code {
      OK = 0;
      FAIL = 1;
    }
  }
}

service Service {
  rpc Call (V1.Request) returns (V1.Response);
}

prerequisite:

"requestPredicates": {
  "order": { "==": "ORDER_ASC" }
}

@mockingbird/maintainers