WebSocketClient::set_ca_cert_path took a single path and create_stream()
hardcoded an empty directory when calling detail::load_client_ca_config, while
ClientImpl has always accepted (ca_cert_file_path, ca_cert_dir_path = ""). Give
WebSocketClient the same signature and store the directory, so both clients
configure CA loading identically. The one-argument form is unchanged for
callers.
Also note at both call sites why the "load the CA config once" guard differs:
SSLClient needs call_once because one client serves concurrent requests, and
WebSocketClient does not because connect() is not safe to call concurrently
anyway.