Add generic constraint for AddHttpApplication (#567)

This adds a generic constraint for HttpApplication for one of the overloads of `AddHttpApplication`. The other overload has it, but this one does not. Things will fail at run time if someone is using it with a non-`HttpApplication` derived type, so this will move error to compile time.

Author: twsouthwick

src/Microsoft.AspNetCore.SystemWebAdapters.CoreServices/HttpApplication/HttpApplicationExtensions.cs

@@ -61,6 +61,7 @@ private static ISystemWebAdapterBuilder AddHttpApplicationInternal(this ISystemW
     }
 
     public static ISystemWebAdapterBuilder AddHttpApplication<TApp>(this ISystemWebAdapterBuilder builder)
+        where TApp : HttpApplication
     {
         ArgumentNullException.ThrowIfNull(builder);