feat(lib/challenge): HTTP meta refresh challenge method (#623)

* feat(lib/challenge): HTTP meta refresh challenge method

Closes #95

This challenge method enables users that don't (or won't) support
JavaScript to pass Anubis challenges. It works by using HTML meta
refresh directives to ensure that the client is a browser.

This is OFF by default. In order to enable it, an administrator MUST
choose to make the default challenge method `metarefresh`.

TODO(Xe):

- [ ] Documentation on this challenge method
- [ ] Amend wording around Anubis being a proof of work proxy in the docs
- [ ] Add configuration file syntax for the default challenge method and settings
- [ ] Test with early customers

Signed-off-by: Xe Iaso <me@xeiaso.net>

* chore: spelling

Signed-off-by: Xe Iaso <me@xeiaso.net>

* fix(lib/challenge/metarefresh): use this value of err

Signed-off-by: Xe Iaso <me@xeiaso.net>

* docs: add metarefresh challenge info, Web AI Firewall Utility

Signed-off-by: Xe Iaso <me@xeiaso.net>

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Signed-off-by: Xe Iaso <me@xeiaso.net>

.github/actions/spelling/expect.txt

@@ -149,6 +149,7 @@ maintainership
 malware
 mcr
 memes
+metarefresh
 metrix
 mimi
 minica