V2 - Replace custom HTTP signing code with RFC 9421 library

Motivation The project implements custom HTTP digital-signature logic in digital_signatures.py (canonicalization, signature headers, content digests). This is security-sensitive and unnecessary to maintain in-house.

Goal Refactor digital_signatures.py to use an RFC 9421-compliant HTTP message-signature library, while keeping eBay-specific signing requirements unchanged.

Recommended Technology requests-http-signature https://github.com/pyauth/requests-http-signature

Outcome Improved correctness, reduced security risk, and less non-core maintenance.