Changeset 43768 for branches/5.0/src/wp-includes/rest-api/endpoints/class-wp-rest-autosaves-controller.php

REST API: Introduce Autosaves controller and endpoint.

• Adds WP_REST_Autosaves_Controller which extends WP_REST_Revisions_Controller.
• Autosaves endpoint is registered for all post types except attachment because even post types without revisions enabled are expected to autosave.
• Because setting the DOING_AUTOSAVE constant pollutes the test suite, autosaves tests are run last. We may want to improve upon this later.

Props adamsilverstein, aduth, azaozz, danielbachhuber, rmccue.
Fixes #43316.