Update schema with historical average data for parking structure.

schema.graphqls

@@ -30,6 +30,20 @@ type ParkingStructure {
   coordinates: Coordinates!
   address: String!
   updatedTime: DateTime
+
+  historicalAverages(input: HistoricalParkingAverageQueryInput): [HistoricalParkingAverageQueryResult!]
+}
+
+type HistoricalParkingAverageQueryResult {
+  from: DateTime!
+  to: DateTime!
+  averageSpotsAvailable: Int!
+}
+
+input HistoricalParkingAverageQueryInput {
+  from: DateTime!
+  to: DateTime!
+  intervalMs: Int!
 }
 
 type Route {