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/Forward.h

@@ -475,6 +475,10 @@ namespace Web::PerformanceTimeline {
 class PerformanceEntry;
 }
 
+namespace Web::PermissionsPolicy {
+class AutoplayAllowlist;
+}
+
 namespace Web::Platform {
 class Timer;
 }