LibWeb: Make Fetch::Infrastructure::{Request,Response} ref-counted

With the addition of the 'fetch params' struct, the single ownership
model we had so far falls apart completely.

Additionally, this works nicely for FilteredResponse's internal response
instead of risking a dangling reference.

Replacing the public constructor with a create() function also found a
few instances of a Request being stack-allocated!

Userland/Libraries/LibWeb/HTML/NavigationParams.h

@@ -23,10 +23,10 @@ struct NavigationParams {
     String id;
 
     // null or a request that started the navigation
-    OwnPtr<Fetch::Infrastructure::Request> request;
+    RefPtr<Fetch::Infrastructure::Request> request;
 
     // a response that ultimately was navigated to (potentially a network error)
-    NonnullOwnPtr<Fetch::Infrastructure::Response> response;
+    NonnullRefPtr<Fetch::Infrastructure::Response> response;
 
     // an origin to use for the new Document
     Origin origin;