Fetching packages via dnslink

[victorb]

As we always have one root-hash available for the registry we currently have cached, we can also set a DNS record (_dnslink.npm.open-registry.dev) to that value, and have any IPFS gateway being able to fetch the content.

Essentially, we just have to on every change to the registry hash, update a DNS TXT record for this to work.

[victorb]

Also gives us the benefit that we can shield the application from serving incoming requests, as we can make those go directly to the gateway instead of via the app (loadbalancer > ipfs instead of loadbalancer > app > ipfs)