UiCustomFieldsProvider
Interface for providing custom fields for contact and customer contexts.
This interface is used to retrieve extra custom fields that won't be displayed in the UI, but will be appended to the contact and customer contexts when they are set and sent to the backend.
The default internal implementation returns empty maps.
Integration is responsible for providing only valid custom fields key&value pairs, invalid values will cause runtime errors reported by the backend and the SDK.
The fields will be queried from the background thread.