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

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

```diff @@ 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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