New API Endpoints: Custom Field - Field Groupings

Maintain Custom Fields in groups by adding a Custom Field to a Field Group:

POST: https://developers.activecampaign.com/reference/add-custom-field-to-field-group

GET: https://developers.activecampaign.com/reference/get-custom-field-field-group
PUT: https://developers.activecampaign.com/reference/update-custom-field-field-group
DELETE: https://developers.activecampaign.com/reference/delete-custom-field-field-group