IPIP: Delegated IPNS over HTTP

[guseggert]

This is a follow-up from #337, see the comment here: https://github.com/ipfs/specs/pull/337/files#r1022991131

We need to be able to delegate IPNS over HTTP in a similar way as with content routing. One starting use case is for network indexers, from @masih:

On the Network Indexer side I am working on a proposal to support IPNS via Network Indexers for example.