Issue 5258 - Implement Filter::As<>() method to make working with filters easier from C++ code

compiled/subscription/UserDefinedSubscription.h

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
  */
 
 #pragma once
 
 #include "Subscription.h"
 #include "../filter/Filter.h"
 #include "../bindings/runtime.h"
 
 class UserDefinedSubscription : public Subscription
 {
 private:
   int mDefaults;
 
 public:
+  static constexpr Type classType = Type::USERDEFINED;
   explicit UserDefinedSubscription(const String& id);
   bool BINDINGS_EXPORTED IsDefaultFor(const Filter& filter) const;
   void BINDINGS_EXPORTED MakeDefaultFor(const Filter& filter);
   bool BINDINGS_EXPORTED IsGeneric() const
   {
     return mDefaults == 0;
   }
   void BINDINGS_EXPORTED InsertFilterAt(Filter& filter, unsigned pos);
   bool BINDINGS_EXPORTED RemoveFilterAt(unsigned pos);
   OwnedString BINDINGS_EXPORTED Serialize() const;
 };
-
-template<>
-inline UserDefinedSubscription* Subscription::As<UserDefinedSubscription>()
-{
-  if (mType != Type::USERDEFINED)
-    return nullptr;
-
-  return static_cast<UserDefinedSubscription*>(this);
-}