An interval of a CustomPacingCurve. A custom pacing goal contains a start time and an
amount. The goal will apply until either the next custom pacing goal's getStartTime or
the line item's end time if it is the last goal.
The start date and time of the goal. This field is required unless useLineItemStartDateTime is true.
Whether the LineItem.startDateTime should be used for the start date and time of this
goal. This field is not persisted and if it is set to true, the startDateTime field
will be populated by the line item's start time.