REST API: Don't add fields to object when not included in `?_fields=`.

In [43087], we improved REST API performance by only rendering the fields specified in the request. Similarly, any fields registered with `register_rest_field()` should only be rendered when included in `?_fields=`.

Props dlh, danielbachhuber.

Merges [43736] to trunk.

Fixes #45099.
Built from https://develop.svn.wordpress.org/trunk@43986


git-svn-id: https://core.svn.wordpress.org/trunk@43818 1a063a9b-81f0-0310-95a4-ce76da25c4cd