refactor(http-callback): make Http Callback class override-able (#79)

This commit makes `OnBeforeHttpRequestEventHandler` and `OnAfterHttpResponseEventHandler` methods virtual. This allows SDKs to override these methods and invoke user-overridden callbacks methods provided in the SDK class.

Closes #78

APIMatic.Core/Types/HttpCallBack.cs

@@ -24,7 +24,7 @@ public class HttpCallBack
         /// BeforeHttpRequestEventHandler.
         /// </summary>
         /// <param name="request">Http Request.</param>
-        public void OnBeforeHttpRequestEventHandler(CoreRequest request)
+        public virtual void OnBeforeHttpRequestEventHandler(CoreRequest request)
         {
             Request = request;
         }
@@ -34,7 +34,7 @@ public void OnBeforeHttpRequestEventHandler(CoreRequest request)
         /// </summary>
         /// <param name="source">Http Client.</param>
         /// <param name="response">Http Response.</param>
-        public void OnAfterHttpResponseEventHandler(CoreResponse response)
+        public virtual void OnAfterHttpResponseEventHandler(CoreResponse response)
         {
             Response = response;
         }