Add cookie subject extractor

[nothing0012]

Description

Add cookie subject extractor, specifically trying to work with https://developers.cloudflare.com/cloudflare-one/identity/authorization-cookie/application-token/, and assuming that zero trust already did the validation of the jwt, and in this PR, we simply need to extract the subject (i.e. email from the cookie).

Motivation and Context

How Has This Been Tested?

unit test.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

Merging #536 (dd2a603) into main (347a36d) will increase coverage by 0.06%.
The diff coverage is 92.85%.

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@@            Coverage Diff             @@
##             main     #536      +/-   ##
==========================================
+ Coverage   80.96%   81.02%   +0.06%     
==========================================
  Files          28       28              
  Lines        2222     2235      +13     
==========================================
+ Hits         1799     1811      +12     
- Misses        334      335       +1     
  Partials       89       89              

Impacted Files	Coverage Δ
pkg/handler/subject.go	96.55% <92.85%> (-3.45%)	⬇️

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