feat(api): add public APIs for headers

[RedstoneWizard08]

This is a small change, which adds public APIs for the same request header methods used under the hood by the standard CLI server.

This was done for my PR to zoraxy adding native Anubis support.

(Note: I only noticed I had to sign my commits after I made the initial one, so I redid it with a new signed commit and force-pushed. Hopefully that works.)

Checklist:

• Added a description of the changes to the [Unreleased] section of docs/docs/CHANGELOG.md
• Added test cases to the relevant parts of the codebase
• Ran integration tests npm run test:integration (unsupported on Windows, please use WSL)
• All of my commits have verified signatures

[RedstoneWizard08]

Note: Are tests necessary to create here? This is simply exposing an already existing API to the public.