APQ: Initial request should have a status 404 (Not Found), otherwise proxies could cache it
I see the APQ initial request, returning:

```
{"errors":[{"message":"PersistedQueryNotFound","extensions":{"code":"PERSISTED_QUERY_NOT_FOUND"}}]}
```
with status code 200 (OK)

Should it not actually have a status code of 404? So that a proxy typicly will NOT cache it.

Currently (Apollo v4.3.3) it has a status code of 200 (as usual).
This results in caching Proxies caching it, thus always having APQ clients fall back to a second call with the actual query, and thus breaking the beauty of APQ with caching Proxies.
