Enables location and gear testing via API

Adds query parameters for latitude and longitude to the `/TestStartParking` endpoint, allowing to simulate different locations.

Adds logic to set the gear to "P" if the received value is null to cover edge cases.

Ensures asynchronous message processing by awaiting the result of ProcessMessage to prevent potential race conditions.

Source/ProofOfConcept/Services/MQTTClient.cs

@@ -35,7 +35,7 @@ public class MQTTClient : IHostedService
             
                     string? message = Encoding.UTF8.GetString(e.ApplicationMessage.Payload);
                     logger.LogInformation("Message received: {Message}", message);
-                    messageProcessor.ProcessMessage(vin, field.ToLowerInvariant(), message.StripQuotes());
+                    messageProcessor.ProcessMessage(vin, field.ToLowerInvariant(), message.StripQuotes()).GetAwaiter().GetResult();
                 }
                 else
                     logger.LogWarning("Topic not passed to message processor: {Topic}", topic);