* 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.
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danielbachhuber