feat: Implement CGGMP24 key share refresh protocol (#162)

[gyanranjanpanda]

Implement Key Refresh protocol (Fixes #162)

Description

This PR implements the Key Share Refresh protocol as requested in #162, ensuring the long-term security of the secret key by updating individual key shares without changing the underlying secret key.

The implementation covers:

1. Non-Threshold (n-out-of-n): Compliant with Figure 7 of the CGGMP24 paper. Uses a zero-sum vector trick where each party distributes randomly sampled scalars summing to zero. Includes P2P unicast exchange and exact Feldman VSS verification over the $G$ curve, completing with a valid Schnorr proof of knowledge of the new secret share.
2. Threshold (t-out-of-n): Compliant with Appendix F.1.1. Employs zero-sharing polynomials leveraging identical messaging rounds and evaluations but correctly isolating degree-$t-1$ bounds across public VSS polynomials.

Key Changes

• Extracts the refresh logic out to cggmp24-keygen/src/key_refresh_non_threshold.rs and cggmp24-keygen/src/key_refresh_threshold.rs.
• Decouples refresh errors from Bug/KeygenError via separate RefreshBug and RefreshAborted enums to keep upstream Keygen cleaner.
• Ensures all new algorithms use the correct generic-ec 0.5 primitives (e.g., using *secret_scalar.as_ref() for Scalar bounds and .value(&Scalar::zero()) without trait ambiguity).
• Both entry points and builders are re-exported into the main cggmp24 crate alongside Keygen variants.
• Comprehensive integration tests at tests/tests/it/key_share_refresh.rs checking validity mapping across multiple rounds, unaltered secrets with altered shares, threshold min-signer limits, and cross-epoch share incompatibility.

This provides the core cggmp24::keygen::key_refresh functionality as requested, without the legacy aux info elements intermingled into these modules.