REST API: Render only additional fields specific to request when _fields= is used

[dlh]

Follow-up to #43874.

#43874 caused only requested _fields to be processed within the core REST controllers' prepare_item_for_response().

However, callbacks added with register_rest_field() still fire during WP_REST_Controller::add_additional_fields_to_object() regardless of the requested _fields.

The attached patch would add a check for the requested fields to add_additional_fields_to_object() before firing the callbacks.

As suggested in Slack, I'm setting the milestone for this ticket to 5.0: ​https://wordpress.slack.com/archives/C02RQC26G/p1539604416000100.

[danielbachhuber]

In 43736:

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.
Fixes #45099.

[danielbachhuber]

Reopening for merge to trunk.

[desrosj]

In 43986:

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.