Add a predefined union type `ProxyOr[T]`

### Describe the Request

Naming TBD, but it would essentially be:
```python
ProxyOr: TypeAlias = Union[Proxy[T], T]
```
We already internally have this (`ProxyType`), but it'd be nice to expose for users now that the mypy plugin treats `Proxy[T]` and `T` equivalently.

### Sample Code

_No response_