Refactor header key management

- Introduced a new `HeaderKeys` struct in `Constants.cs` to centralize the management of header keys, including a constant for `RequestVerificationToken`.
- Updated `XperienceOutputCachePolicy.cs` to utilize the `RequestVerificationToken` from `HeaderKeys` struct, replacing the hardcoded string. This change improves code maintainability and consistency.

src/XperienceCommunity.OutputCache/Constants.cs

@@ -13,6 +13,11 @@ internal readonly struct ItemKeys
             internal const string DependencyKeys = "DependencyKeys";
         }
 
+        internal readonly struct HeaderKeys
+        {
+            internal const string RequestVerificationToken = "RequestVerificationToken";
+        }
+
         internal readonly struct CacheSegments
         {
             /// <summary>

src/XperienceCommunity.OutputCache/Policies/XperienceOutputCachePolicy.cs

@@ -122,7 +122,7 @@ private static bool AttemptOutputCaching(OutputCacheContext context)
             }
 
             // Verify existence of anti-forgery token
-            if (!StringValues.IsNullOrEmpty(request.Headers["RequestVerificationToken"]))
+            if (!StringValues.IsNullOrEmpty(request.Headers[Constants.HeaderKeys.RequestVerificationToken]))
             {
                 return false;
             }