[Core API] Resource-specific custom fields

Front is introducing resource-specific custom fields. In addition to Contacts, custom fields can now be added to Inboxes and Teammates.

Added

• GET /contacts/custom_fields endpoint to list the custom fields are applicable to Contacts.
• GET /inboxes/custom_fields endpoint to list the custom fields are applicable to Inboxes.
• GET /teammates/custom_fields endpoint to list the custom fields are applicable to Teammates.
• PATCH /inboxes/:inbox_id endpoint to update an inbox name and custom fields.

Changed

• PATCH /teammates/:teammate_id now supports the property custom_fields to update the custom fields of a teammate.
• POST /inboxes now support the property custom_fields to create an inbox with custom fields.

Deprecated

• GET /custom_fields. Developers should use GET /contacts/custom_fields instead.