Add Subprotocol Negotiation headers upon accepting WebTransport connection

[englishm]

Subprotocol Negotiation offers a way to denote that we're running MoQ over WebTransport (similar to what we can do with ALPN in the case of raw QUIC).

I propose we adopt the same string here as for an ALPN value (the current draft says 'moq-00').

We can add support for this on the server side now, but it won't be useful until the browser WebTransport constructor has a way to signal this, too.

Relates to: kixelated/moq-js#94