Issue title: De-Fusion: WP REST API Extensions
 Issue body: ### Impacted plugin

Jetpack

### What

WP REST API Extensions

### How

```
"$plugin_root/_inc/lib/core-api/class-wpcom-rest-field-controller.php" => "$wpcom_root/wp-content/rest-api-plugins/centralized/class-wpcom-rest-field-controller.php",
```
 Comments: 
 Comment 0: Update diff: D88846-code (note about `/tests/php/core-api/test_class-wpcom-rest-field-controller.php` in there)
Testing: make sure tests are passing and observe post-deploy statuses; this should be a straightforward swap.
 Comment 1: Unsure how to test this other than making sure the new location is loading, but let's make sure we're around to monitor this one. 
 Comment 2: deployed r254212-wpcom
{'source': 'GithubIssues', 'repo_name_with_owner': 'Automattic/jetpack', 'issue_id': 1315352535, 'repo_stars': 1619.0}