Add feature settings REST API

[JakePT]

Description of the Change

Adds a REST API endpoint at /wp-json/elasticpress/v1/features for updating feature settings. The endpoint uses the settings schema added to features by #3655 to provide validation for updating the ep_feature_settings via a PUT request.

This PR also creates a Dashboard screen class to register the endpoint (for consistency with Sync). The name dashboard was for consistency with the dashboard-page.php file used to render it.

How to test the Change

1. Make an OPTIONS request to /wp-json/elasticpress/v1/features. The feature settings and their types and possible values should be shown as supported args.
2. Copy the saved ep_feature_settings option from the database and unserialize it as JSON. Make modifications and send it as the body in a PUT request to /wp-json/elasticpress/v1/features. Valid changes according to the definitions in the OPTIONS response should update the feature settings, and be reflected in the admin, while changes that are invalid should result in a 400 error.

Changelog Entry

Added - Added a REST API endpoint at elasticpress/v1/features for updating feature settings.

Credits

Props @JakePT

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[felipeelia]

Everything looks good @JakePT, the only thing I think we need to change is a @since tag.