fix(security): fix cookies leakage between different users

[WSH032]

Summary

• fix security vulnerabilities of cookies leakage between different users

Through this PR, fastapi-proxy-lib takes over the AsyncClient cookies argument.

For example, the following do not work anymore.

ForwardHttpProxy(AsyncClient(cookies={"foo": "bar"}))

But normal cookie sending and receiving are not affected.

Security-advisories: GHSA-7vwr-g6pm-9hc8

Checklist

• I've read CONTRIBUTING.md.
• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5836696) 96.71% compared to head (305a2ca) 96.74%.
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #10      +/-   ##
==========================================
+ Coverage   96.71%   96.74%   +0.02%     
==========================================
  Files           9        9              
  Lines         457      461       +4     
  Branches       66       67       +1     
==========================================
+ Hits          442      446       +4     
  Misses          9        9              
  Partials        6        6              

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