Fetch dynamic JWK based on path

[kixelated]

#809 added the ability to download key files from a remote.

Ideally, we could fetch a JWK given a path. So if someone tries to publish to cdn.moq.dev/hang.live, then we would fetch the key for hang.live unless it's in cache already (and hasn't expired).

This would unlock the ability to run a MoQ service using an API server to fetch keys based on the customer. It would also avoid redownloading keys every 30s if we don't actually need them. Cache control headers could probably handle that for us.

@einrobin 👀

[kixelated]

If somebody implements this, I will totally vibe code an endpoint that allocates keys for cdn.moq.dev access. That way we don't all have to share /anon and make everything public.