LibWeb+LibWebView+WebContent: Add APIs to manage an autoplay allowlist

The spec defines a Permissions Policy to control some browser behaviors
on a per-origin basis. Management of these permissions live in their own
spec: https://w3c.github.io/webappsec-permissions-policy/

This implements a somewhat ad-hoc Permissions Policy for autoplaying
media elements. We will need to implement the entire policy spec for
this to be more general.

Userland/Libraries/LibWeb/PermissionsPolicy/AutoplayAllowlist.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+#include <AK/Variant.h>
+#include <AK/Vector.h>
+#include <LibWeb/Forward.h>
+#include <LibWeb/PermissionsPolicy/Decision.h>
+
+namespace Web::PermissionsPolicy {
+
+class AutoplayAllowlist {
+public:
+    static AutoplayAllowlist& the();
+
+    Decision is_allowed_for_origin(DOM::Document const&, HTML::Origin const&) const;
+
+    void enable_globally();
+    ErrorOr<void> enable_for_origins(ReadonlySpan<String>);
+
+private:
+    AutoplayAllowlist();
+    ~AutoplayAllowlist();
+
+    using Patterns = Vector<HTML::Origin>;
+    struct Global { };
+
+    Optional<Variant<Patterns, Global>> m_allowlist;
+};
+
+}