Update to latest HTTP Working Group spec

The latest version of the spec is quite a bit different:
- https://www.ietf.org/archive/id/draft-ietf-httpbis-message-signatures-08.html
- https://httpsig.org/

I'm trying to use this with a js library (https://github.com/dhensby/node-http-message-signatures/), but can't since the specs don't agree.

I can try to work on a PR if I have time, but wanted to check in and see if this project was still maintained and if you'd be open to supporting the newer spec.

The issue references updating to the latest HTTP Message Signatures spec (draft-ietf-httpbis-message-signatures-08). Start by reading the linked spec and the httpsig.org overview. Examine the existing library code to understand the current signing/verification implementation. Compare with the node-http-message-signatures library for interoperability. The work involves updating the core signing logic to match the new spec, which may affect multiple classes.

索引モデルが issue の本文から書いたものです。