Update feedback structure for overrideReason

[dennispatterson]

When an override reason is chosen on a card dismissal, updating the structure sent to reflect changes in the spec: https://cds-hooks.org/specification/current/#overridereason

Verified by running locally and observing the following sent to a CDS Service:

{
   "feedback":[
      {
         "card":"3665d2ff-4dde-47de-85d2-03b56deca418",
         "outcome":"overridden",
         "outcomeTimestamp":"2020-08-25T21:18:16.375Z",
         "overrideReason":{
            "reason":{
               "code":"patient-requested-brand",
               "system":"http://terminology.cds-hooks.org/CodeSystem/OverrideReasons"
            }
         }
      }
   ]
}