Short-circuit HEAD methods in Core controllers

[TimothyBlynJacobs]

The REST API has built-in support for responding to a HEAD request. If no callback is specifically registered for the method, the server will fallback to the associated GET handler. However, this means that the server is throwing away work when no response body is needed.

GET routes can be adapted to check if the WP_REST_Request::get_method is HEAD and if so skip preparing the response body. For single item routes, this is pretty straightforward.

It is a little more nebulous for collection routes. We still need to provide any headers like pagination which are currently prepared after the response body is generated.

I think we'd want to extract the pagination logic to it's own method. Then, before making the database query,
adjust the query to only return ids. This will give us enough information to build the pagination headers and allow us to send the HEAD response.

See ​https://github.com/WordPress/gutenberg/pull/43703