Adding VRF-based computation of labels

[Jasleen1]

We want to use a VRF to compute labels in seemless_directory.rs and then verify these VRFs in lookup_verify, key_history_verify etc.

See: https://crates.io/crates/vrf for one option. Below is a starting checklist of things to consider:

• We will need to convert the output of the VRF into NodeLabel type. We may want to change the NodeLabel type as a result, or implement a trait instead.
• VRF keys need to be stored in some sort of config file, so that the server can generate VRF values.
• In the Java implementation of SEEMless, we used BouncyCastle, and at the time, we had to hash to the curve from strings/bytearrays. We need to hash to the correct space in this case as well.
• If we hash to a curve and later hash the output value, we should be careful about the security guarantees offered by the various parameter sizes. Would be helpful to add documentation for this.

[slawlor]

Implementation merged based on ECVRF-ED25519-SHA512-TAI reference. See linked PRs.

@Jasleen1 what's left on the checklist to be done with the landing of VRFs?

[kevinlewi]

I believe this should be done and this can be closed (finally!), but @Jasleen1 to confirm