REST API: Declare unfiltered_html capability in links.

Because user capabilities can be modified at runtime, the REST API needs to expose them in some evaluated but declarative manner for clients to interpret. JSON Hyper Schema targetSchema provides an appropriate paradigm for doing so.

Merges [43682] to trunk.

Props timothyblynjacobs.

Fixes #45014.

Built from https://develop.svn.wordpress.org/trunk@43974


git-svn-id: https://core.svn.wordpress.org/trunk@43806 1a063a9b-81f0-0310-95a4-ce76da25c4cd