Implement Signing HTTP Messages with structured field values

[aljones15]

Spec Implementation Issue

• [x] Add changelog entry
• [x] Add tests for createSignatureHeader
• [x] Add more tests for parseRequest
• [x] Review spec for missing normative statements

EDIT: this implementation was made against version 4 of the major rewrite of the spec, since May 2021 at least 5 additional revisions of the spec have been made. To see the latest spec please go here: https://datatracker.ietf.org/doc/draft-ietf-httpbis-message-signatures/

[mattcollier]

@aljones15 looks like some merge conflicts have cropped up sometime in the last 3 years. :)