Automatically wrap objects with the correct ProxyModel when passed to a link with props. (#903)

Given a schema:
```
type Target;
type SourceWithProp {
    target: Target {
        lprop: int64;
    };
};
```

The following code will now sync correctly and pass assertions:
```
target = default.Target()
self.client.save(target)

source = default.SourceWithProp(target=target)
self.client.sync(source)

assert hasattr(source.target, '__linkprops__')
assert hasattr(source.target.__linkprops__, 'lprop')
assert source.target.__linkprops__.lprop is None
```

Previously, users would have to wrap the target using `default.SourceWithProp.target.link()` or a `ValueError` would be raised.

Similar behaviour has also been added to multi links.