Separate out GetExternalIPAddress helper function

De-duplicates the two inline implementations and prepares it for use from Android.

Source/Core/DolphinQt/NetPlay/NetPlayDialog.cpp

@@ -30,7 +30,7 @@
 #endif
 
 #include "Common/Config/Config.h"
-#include "Common/HttpRequest.h"
+#include "Core/NetPlayCommon.h"
 #include "Common/Logging/Log.h"
 #include "Common/TraversalClient.h"
 
@@ -550,17 +550,8 @@ void NetPlayDialog::show(std::string nickname, bool use_traversal)
 
 void NetPlayDialog::ResetExternalIP()
 {
-  m_external_ip_address = Common::Lazy<std::string>([]() -> std::string {
-    Common::HttpRequest request;
-    // ENet does not support IPv6, so IPv4 has to be used
-    request.UseIPv4();
-    Common::HttpRequest::Response response =
-        request.Get("https://ip.dolphin-emu.org/", {{"X-Is-Dolphin", "1"}});
-
-    if (response.has_value())
-      return std::string(response->begin(), response->end());
-    return "";
-  });
+  m_external_ip_address =
+      Common::Lazy<std::string>([]() -> std::string { return NetPlay::GetExternalIPAddress(); });
 }
 
 void NetPlayDialog::UpdateDiscordPresence()