[DO NOT MERGE] fix: Added signature_share_node for mitigating rogue attacks

[ChaoticTempest]

addresses #120

This adds in a new signature_share_node endpoint to share the signature of the nodes signed message: NEAR. The leader node will aggregate all these signatures and send them out along side the public keys to each sign node, and each sign node will do their separate verification before accepting the public keys as true set of public keys.

Probably need to store these signatures to GCP as well, just so we can do the verification again when it gets restored, but will leave that for later if needed and if this design is good enough

Also had to change the accept_pk_set endpoint request to take in the signatures, so hopefully upgrading isn't a huge issue for partners

[ChaoticTempest]

Should be good to be reviewed again. This now uses the Rogue Attack section of the math found in https://hackmd.io/-tXnfndwS4a-QLM_UvtRRw which is implemented in the multi-party-eddsa repo: DavidM-D/multi-party-eddsa#1

[ChaoticTempest]

TODO: need to supply a not known ahead of time hash function to add_signature_parts_hashed so that each sign node can choose a hash function without the leader knowing what the hash function is

[DavidM-D]

This seems a bit more complicated than I would have expected.

When a node says it has a key, we need it to prove it actually does by signing something with said key, that can be an empty or a constant string.

Is it possible you've been reading about rogue key attacks for threshold signatures rather than aggregate signatures?

[ChaoticTempest]

Closing this in favor of #169