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/repositories/parking/ParkingGetterRepository.ts

@@ -1,14 +1,14 @@
 import { IParkingStructure } from "../../entities/ParkingRepositoryEntities";
 
 export interface ParkingStructureCountOptions {
-  startUnixEpochMs: number;
-  endUnixEpochMs: number;
+  startUnixEpochMs: Date;
+  endUnixEpochMs: Date;
   intervalMs: number;
 }
 
 export interface HistoricalParkingAverageQueryResult {
-  fromUnixEpochMs: number;
-  toUnixEpochMs: number;
+  fromUnixEpochMs: Date;
+  toUnixEpochMs: Date;
   averageSpotsAvailable: number;
 }