support Alt-Svc layer + cache #1145

This will be picked up by me, right after H3 support is full and complete (#14)

We already have implemented the typed header, and we also already support SVCB/HTTPS records + connectors based on it. So that's the more important one for pre-knowledge. Alt-Svc is cheryr on cake for future requests in case a server doesn'rt do DNS properly but does suppot Alt-Svc.... (todo: lookup if this cna also be in html, or just in response header as the latter we do have suport for)