[FIX] Enable API access key auth for custom_field actions

Enabled authentication via API access key for:

* ```/api/v2/custom_fields.:format```
* ```/api/v2/custom_fields/:id.:format```
* ```/api/v2/projects/:project_id/planning_element_custom_fields.:format```

Previously only basic auth worked for those.

[Issue #2708](https://www.openproject.org/issues/2708)