Change ParkingStructureCountOptions and HistoricalParkingAverageQueryResult to use Date objects

This matches the behavior of `updatedTime` on shuttle objects. When returning API data, dates are converted into milliseconds since Epoch by the DateTime scalar implementation.

src/entities/ParkingRepositoryEntities.ts

@@ -9,7 +9,7 @@ export interface IParkingStructure extends IEntityWithTimestamp, IEntityWithId {
 }
 
 export interface IParkingStructureTimestampRecord {
-  timestampMs: number;
+  timestampMs: Date;
   id: string;
   spotsAvailable: number;
 }