This helps with #2067 and clarifies the relationship between site config and external service config.
Test plan:
- Verify that equivalent Go `./schema` code is codegenned (vs. before this PR, by looking at the diff).
- For the web app, ensure that in the site admin external service Monaco editors for each kind of service, you get hovers and validation as before.
- For the backend, ensure that server-side validation occurs when you try to save external service JSON in site admin. (This is covered by Go unit tests.)
