REST API: Link to the REST route for the currently queried resource.

This allows for programatically determining the REST version of the current page. The links also aid human discovery of the REST API in general.

Props dshanske, tfrommen, TimothyBlynJacobs.
Fixes #49116.

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git-svn-id: https://core.svn.wordpress.org/trunk@48042 1a063a9b-81f0-0310-95a4-ce76da25c4cd