Returns the current communication preferences for a customer, or a single preference by its identifier.

Exactly one of customer_ext_id or preference_id must be supplied.

• When queried by customer_ext_id, all preferences (across groups and channels) for the customer are returned. An empty array is returned if the customer has no preferences or is not known to Flex.
• When queried by preference_id, a single preference matching the supplied id is returned, or 404 if no preference matches.

Each preference reflects the latest opt-in or opt-out state for a given customer, group and channel combination.