feat: basic authenticator (#254)

[viters]

Basic authentication is sometimes useful for content that should not be publicly accessible, but does not need the level of security of other authentication methods. Ex. in-development web pages, temporarily hidden content.

It should work in combination with WWW-Authenticate error handler.

Related issue(s)

#254
This issue is quite old, but I today stumbled upon situation where basic authn could be useful to me - so I've decided to implement it.

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got the approval (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further comments

I will prepare a PR to docs if this feat gets approved.

[codecov]

Codecov Report

Merging #1044 (7444f08) into master (7017fdf) will decrease coverage by 0.68%.
The diff coverage is 72.50%.

❗ Current head 7444f08 differs from pull request most recent head d1b382c. Consider uploading reports for the commit d1b382c to get more accurate results

@@            Coverage Diff             @@
##           master    #1044      +/-   ##
==========================================
- Coverage   78.71%   78.03%   -0.69%     
==========================================
  Files          83       84       +1     
  Lines        3871     4033     +162     
==========================================
+ Hits         3047     3147     +100     
- Misses        554      605      +51     
- Partials      270      281      +11     

Impacted Files	Coverage Δ
pipeline/authn/authenticator_basic.go	71.79% <71.79%> (ø)
driver/registry_memory.go	90.12% <100.00%> (+0.04%)	⬆️
internal/cloudstorage/setup.go	60.13% <0.00%> (-9.10%)	⬇️
internal/driver.go	100.00% <0.00%> (+22.22%)	⬆️

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[Davincible]

@aeneasr