docs: Go sdk custom client documentation

docs/docs/clients/server-side.mdx

@@ -1561,6 +1561,18 @@ client := flagsmith.NewClient(os.Getenv("FLAGSMITH_SERVER_SIDE_ENVIRONMENT_KEY")
                 c.client.SetProxy(proxyURL)
             }
         }
+
+        // WithRestyClient allows you to provide a custom resty client for making HTTP requests.
+        // This gives you more control over the HTTP client configuration.
+        // Only one of resty or HTTP custom client can be provided
+        // Can not be used simultaneously with Client related options (WithRequestTimeout, WithRetries, WithCustomHeaders, WithProxy)
+        flagsmith.WithRestyClient(restyClient)
+
+        // WithHTTPClient allows you to provide a custom http client for making HTTP requests.
+        // This is useful when you need to customize the underlying HTTP client behavior.
+        // Only one of resty or HTTP custom client can be provided
+        // Can not be used simultaneously with Client related options (WithRequestTimeout, WithRetries, WithCustomHeaders, WithProxy)
+        flagsmith.WithHTTPClient(httpClient)
 )
 ```