Make querying `isOneOf` optional in Introspection query (#1737)

Including it by default makes introspection fail on servers that do not support it:

```
Cannot query field "isOneOf" on type "__Type".
```

Thus, I added a configuration option `typeIsOneOf` which works analogous to `directiveIsRepeatable`.

Update input object type docs and prepare v15.21.0

#1715

Prepare v15.20.0 release

Reset static caches of instances using standard types when overriding…

… them