Support for registering HEAD callback in REST API

[danielbachhuber]

From #34837

It should be possible to register an optional HEAD callback for an endpoint. However, if no HEAD callback is provided, the REST API should fall back to the GET callback.

Discussion: ​https://wordpress.slack.com/archives/core/p1449160413002916

[rmccue]

Allow registering HEAD endpoints

[rmccue]

Added patch; this requires registering the HEAD endpoint before the GET one. Essentially internally we're now treating GET as GET,HEAD, so this matches the behaviour of first-match we currently have (that is, if you have two endpoints with POST,PUT and POST respectively, a POST will match the first one).

@danielbachhuber @joehoyle @rachelbaker :thumbsup: on the approach? We'll need to make sure it's documented that GET is treated as GET,HEAD.

[rachelbaker]

Add period at end of inline comment

[rmccue]

In 36535:

REST API: Allow explicit HEAD callbacks.

HEAD callbacks can now be registered independently, with the GET
callback still used as a fallback.

Fixes #34841.