LibWeb: Implement most of the 'Fetching' AOs

This implements the following operations from section 4 of the Fetch
spec (https://fetch.spec.whatwg.org/#fetching):

- Fetch
- Main fetch
- Fetch response handover
- Scheme fetch
- HTTP fetch
- HTTP-redirect fetch
- HTTP-network-or-cache fetch (without caching)

It does *not* implement:

- HTTP-network fetch
- CORS-preflight fetch

Instead, we let ResourceLoader handle the actual networking for now,
which isn't ideal, but certainly enough to get enough functionality up
and running for most websites to not complain.

Userland/Libraries/LibWeb/Fetch/Fetching/PendingResponse.h

@@ -0,0 +1,44 @@
+/*
+ * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Forward.h>
+#include <LibJS/Heap/Cell.h>
+#include <LibJS/SafeFunction.h>
+#include <LibWeb/Fetch/Infrastructure/HTTP/Responses.h>
+
+namespace Web::Fetch::Fetching {
+
+// Non-standard wrapper around a possibly pending Infrastructure::Response.
+// This is needed to fit the asynchronous nature of ResourceLoader into the synchronous expectations
+// of the Fetch spec - we run 'in parallel' as a deferred_invoke(), which is still on the main thread;
+// therefore we use callbacks to run portions of the spec that require waiting for an HTTP load.
+class PendingResponse : public JS::Cell {
+    JS_CELL(PendingResponse, JS::Cell);
+
+public:
+    using Callback = JS::SafeFunction<void(JS::NonnullGCPtr<Infrastructure::Response>)>;
+
+    [[nodiscard]] static JS::NonnullGCPtr<PendingResponse> create(JS::VM&);
+    [[nodiscard]] static JS::NonnullGCPtr<PendingResponse> create(JS::VM&, JS::NonnullGCPtr<Infrastructure::Response>);
+
+    void when_loaded(Callback);
+    void resolve(JS::NonnullGCPtr<Infrastructure::Response>);
+
+private:
+    PendingResponse() = default;
+    explicit PendingResponse(JS::NonnullGCPtr<Infrastructure::Response>);
+
+    virtual void visit_edges(JS::Cell::Visitor&) override;
+
+    void run_callback() const;
+
+    Callback m_callback;
+    JS::GCPtr<Infrastructure::Response> m_response;
+};
+
+}