Internal handling of signature assignment and management

[CPerezz]

It was a nightmare and really error-prone to implement correctly all the steps in https://github.com/privacy-scaling-explorations/zkevm-circuits/blob/6abc33eeb81ffd64ff73f2a84c04203f33c161eb/eth-types/src/sign_types.rs#L100 when we migrated to the latest halo2 and halo2wrong tags.

It will be much better if the secp256k1 crate would handle these kind of things for us.

We could maybe even create a secp256k1-sign crate which handles all that for us and abstracts complexity in the downstream crates.
This would help significantly to reduce code there and also, would place in the correct repo all the signature-related logic.

[CPerezz]

Seolved via #59 when resolved.