refactor: simplify k generation following RFC 6979

[tcoratger]

Summary

This pull request introduces a direct integration of the rfc6979 crate to generate the scalar k according to RFC 6979. The integration is aimed at streamlining the logic within our codebase, ensuring simplicity, and reducing maintenance overheads by relying on established libraries from RustCrypto.

Changes Made

• Integrated the rfc6979 crate to handle the generation of scalar k.
• Refactored relevant sections of the codebase to utilize the new integration.
• Ensured consistency and compliance with RFC 6979 standards.

Motivation

• Simplifying Codebase: By directly leveraging the rfc6979 crate, we simplify the logic related to k generation, making it more concise and maintainable.
• Reduced Maintenance Burden: Depending on established libraries from RustCrypto reduces the need for maintaining custom solutions in our repository, ensuring compatibility and reliability.

[xJonathanLEI]

Rebased to keep branch from becoming staled.