WebSockets: add support for permessage-deflate extension

Add new API to add WebSocket extensions to SoupSession and SoupServer and include an implementation of permessage-deflate extension (see RFC 7692). In the client side, supported extensions are added to the session as sub-features of a new session feature, SoupWebsocketExtensionManager. In the client side, supported extensions are added/removed directly using the new SoupServer API. All functions to negotiate the handshake (client_prepare, client_verify, server_check and server_process) have now a _with_extensions alternative to handle the extensions.