Resources referenced by name but not used directly generate helper methods

[software-dov]

Consider the following:

message Squid {
  option (google.api.resource) = {
    type: "animalia.cephalopoda.com/Squid"
    pattern: "zones/{zone}/squids/{squid}"
  };
  string name = 1;
}

message CreateSquidRequest {
  string name = 1 {
    (google.api.resource_reference) = {
      type: "animalia.cephalopoda.com/Squid"
    }
  };
}

message CreateSquidResponse {}

service SquidService {
  rpc CreateSquid(CreateSquidResponse) returns CreateSquidResponse {
    option (google.api.method_signature) = "name";
  }
}

The Squid resource is never directly referenced as an input or output message type or one of their fields, but we still want to generate resource helper methods for Squid.

Tracking issue for following resource_reference annotations in fields.