Add private fetches for Merkle proofs using Blyss

[menonsamir]

This PR adds the ability to privately fetch Merkle proofs using homomorphic encryption. The underlying service that actually serves the proofs is called Blyss.

The cool part is that the server cannot learn anything about which Merkle proof is being fetched. It's fairly efficient: 128 KB of download and ~2 seconds. The test in this PR uses the Blyss service, but the server can also be self-hosted.

This is just a draft, since still figuring out if this repo is the best place to put this functionality. CC @philsippl