Communicate with network indexers for content routing using reader privacy with double hashing

[BigLep]

Done Criteria

Content routing calls like findProviders to Network Indexers like cid.contact preserve reader privacy with double hashing. Using double hashing is the implicit default.

Why Important

Move IPFS' privacy story forward so that centralized endpoints like network indexers are not even able to know "who is contacting what". This is about getting to a system where we don't have to trust network indexers to respect a client's privacy.

Notes

1. There is a lot still be designed here. This is the Kubo tracking issue so we can link to other work.
2. This is a followup to https://github.com/ipfs/kubo/issues/9454
3. Related docs:
   • https://www.notion.so/pl-strflt/Double-Hashing-for-Privacy-ff44e3156ce040579289996fec9af609
   • https://protocollabs.notion.site/Double-Hashing-and-Indexers-abe2e24aa5974f5ebd7fc42a17f1bd05
   • https://github.com/ipni/specs/pull/5

[BigLep]

There is still open spec work for this (https://github.com/ipfs/specs/pull/421 ) and then the client implementation needs to land in boxo.