New Custom Fields API endpoint

Added a new read-only API endpoint at /api/v1/custom_fields that allows retrieving configured custom field(s). The endpoint supports filtering by:

• Date ranges (created_at and updated_at)
• Field type (e.g., CustomFieldString, CustomFieldSelect, CustomFieldDate)
• Model type (e.g., Application, Child)

Returns custom field attributes including label, model, type, agency_id, created_at, and updated_at.