Allow remote relationships in subscriptions

### Is your proposal related to a problem?

I was surprised to learn today that I can't use remote relationships in subscriptions.

The current implementation (2.8.3) shows an error instead `Remote relationships are not allowed in subscriptions`

Issue #4815 says remote fields should not appear in subscriptions. I _don't_ want that. I want the opposite.

### Describe the solution you'd like

I want to be able to use remote relationships (and the resulting fields) in subscriptions.

### Describe alternatives you've considered

Use regular queries.

### If the feature is approved, would you be willing to submit a PR?

No.

The issue names no files, tests, or entry points. Begin by locating the subscription validation that produces “Remote relationships are not allowed in subscriptions” and its existing tests. Done means remote relationships and their resulting fields work in subscriptions without that error, with coverage for the new behavior.

Written by the indexing model from the issue text.