Fix mockgenerator schema (#424)

Author: waldekmastykarz

dev-proxy-plugins/MockResponses/MockResponse.cs

@@ -10,7 +10,7 @@ public class MockResponse {
     public string Url { get; set; } = string.Empty;
     [JsonPropertyName("method")]
     public string Method { get; set; } = "GET";
-    [JsonPropertyName("nth")]
+    [JsonPropertyName("nth"), JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
     public int? Nth { get; set; }
     [JsonPropertyName("responseCode")]
     public int? ResponseCode { get; set; } = 200;

dev-proxy-plugins/MockResponses/MockResponsePlugin.cs

@@ -23,6 +23,8 @@ public class MockResponseConfiguration {
     [JsonIgnore]
     public bool BlockUnmockedRequests { get; set; } = false;
 
+    [JsonPropertyName("$schema")]
+    public string Schema { get; set; } = "https://raw.githubusercontent.com/microsoft/dev-proxy/main/schemas/v1.0/responses.schema.json";
     [JsonPropertyName("responses")]
     public IEnumerable<MockResponse> Responses { get; set; } = Array.Empty<MockResponse>();
 }