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.