SoupWebsocketExtensionManager is the [iface@SessionFeature] that handles WebSockets extensions for a [class@Session].
A WebsocketExtensionManager is added to the session by default, and normally you don't need to worry about it
at all. However, if you want to disable WebSocket extensions, you can remove the feature from the session with [
[email protected]_feature_by_type] or disable it on individual requests with [[email protected]_feature].