Add generated_slug to REST API autosaves endpoint

[earnjam]

#45017 led to adding generated_slug and permalink_template to the posts controller.

In order to facilitate more timely and accurate editing of permalinks in the new block editor, it would also be helpful to know about any unique slug conflicts by returning the generated_slug value on the autosaves endpoint as well. Otherwise we have to wait until a manual save or publish in order to check the true generated_slug value.

See ​https://github.com/WordPress/gutenberg/pull/12009 for work on this and ​this Slack comment for more details.

[TimothyBlynJacobs]

I see the Gutenberg PR for this has been merged. Does this still need Core changes to the REST API?