Use proper hash to curve algorithm for BLS signatures

[jannikluhn]

To create BLS signatures in G2, we need to first map the message to a point in G2. At the moment, we do this by hashing the message to a string, interpreting it as an integer, and multiplying it by the base point. This is not the proper way to do it.

• The cloudflare library has an issue for this: Hashing to G2 cloudflare/bn256#17
• If EIP-2537 is activated, we should probably switch to the BLS12-381 curve and we can use whatever the Eth2 clients use.
• If we have to implement it ourselves, we have two options:
  • Implement a proper algorithm, e.g., described here: https://datatracker.ietf.org/doc/draft-irtf-cfrg-hash-to-curve/
  • Implement "hash and check" which supposedly is insecure, but should be acceptable for our use case (because the attacker cannot choose the message that's signed) -- see here for a description, but in the context of BLS12-381.

[jannikluhn]

we should figure out if we actually need it or if the current implementation is good enough

[jannikluhn]

not needed anymore since we're likely not going to use the decryptors