For security reasons, browsers are limited in the header fields they can
send when opening a WebSocket connection. Specifically, the macaroon
cannot be sent in the Grpc-Metadata-Macaroon header field as that would
be possible for normal REST requests. Instead we only have the special
field "Sec-Websocket-Protocol" that can be used to transport custom
data. We allow the macaroon to be sent there and transform it into a
proper header field for the target request.
