Framework for retrying failing HTTP requests (#117)

* Retry support

* Adding retry factory decorator

* Added retry initializer

* Added documentation

* Simplified how the back off handler is registered

* Made the back off handler private; Added factor support for exp back off

Author: hiranya911

FirebaseAdmin/FirebaseAdmin.Tests/Util/ErrorHandlingHttpClientTest.cs

@@ -216,6 +216,53 @@ public void NoErrorResponseHandler()
                 () => new ErrorHandlingHttpClient<FirebaseException>(args));
         }
 
+        [Fact]
+        public async Task RetryOnErrorResponse()
+        {
+            var handler = new MockMessageHandler()
+            {
+                StatusCode = HttpStatusCode.ServiceUnavailable,
+                Response = "{}",
+            };
+            var factory = new MockHttpClientFactory(handler);
+            var args = this.CreateArgs(factory);
+            args.RetryOptions = this.RetryOptionsWithoutBackOff();
+            var httpClient = new ErrorHandlingHttpClient<FirebaseException>(args);
+
+            var exception = await Assert.ThrowsAsync<FirebaseException>(
+                async () => await httpClient.SendAndDeserializeAsync<Dictionary<string, string>>(
+                    this.CreateRequest()));
+
+            Assert.Equal(ErrorCode.Internal, exception.ErrorCode);
+            Assert.Equal("Example error message: {}", exception.Message);
+            Assert.Null(exception.InnerException);
+            Assert.NotNull(exception.HttpResponse);
+            Assert.Equal(5, handler.Calls);
+        }
+
+        [Fact]
+        public async Task RetryOnNetworkError()
+        {
+            var handler = new MockMessageHandler()
+            {
+                Exception = new HttpRequestException("Low-level network error"),
+            };
+            var factory = new MockHttpClientFactory(handler);
+            var args = this.CreateArgs(factory);
+            args.RetryOptions = this.RetryOptionsWithoutBackOff();
+            var httpClient = new ErrorHandlingHttpClient<FirebaseException>(args);
+
+            var exception = await Assert.ThrowsAsync<FirebaseException>(
+                async () => await httpClient.SendAndDeserializeAsync<Dictionary<string, string>>(
+                    this.CreateRequest()));
+
+            Assert.Equal(ErrorCode.Unknown, exception.ErrorCode);
+            Assert.Equal("Network error", exception.Message);
+            Assert.Same(handler.Exception, exception.InnerException);
+            Assert.Null(exception.HttpResponse);
+            Assert.Equal(5, handler.Calls);
+        }
+
         [Fact]
         public async Task Dispose()
         {
@@ -256,6 +303,13 @@ private HttpRequestMessage CreateRequest()
             };
         }
 
+        private RetryOptions RetryOptionsWithoutBackOff()
+        {
+            var copy = RetryOptions.Default;
+            copy.BackOffFactor = 0;
+            return copy;
+        }
+
         private class TestHttpErrorResponseHandler : IHttpErrorResponseHandler<FirebaseException>
         {
             public FirebaseException HandleHttpErrorResponse(HttpResponseMessage response, string body)