A custom contact property definition: key, type ("string" | "number") and the fallback_value used when a contact has no value. remove/2 populates id plus deleted.
key
type
"string" | "number"
fallback_value
remove/2
id
deleted
@type t() :: %MillionSend.ContactProperties.ContactProperty{ created_at: term(), deleted: term(), fallback_value: term(), id: term(), key: term(), object: term(), type: term() }