fix: Added support for inline URL username:password proxy auth

[matt-way]

Rationale

Currently the following code fails with a non 200 status tunneling issue:

await fetch(url, {
  dispatcher: new ProxyAgent(`http://<username>:<password>@myproxyurl.com:9000`)
})

This is because an inline username/password is ignored, and no auth header is created. This PR creates a Basic 'proxy-authorization' header if an inline username & password is provided.

Related

https-proxy-agent implements this for node-fetch: https://github.com/TooTallNate/proxy-agents/blob/main/packages/http-proxy-agent/src/index.ts#L102

Changes

• Add proxy-authorization header when an inline username/password is provided (opts.token takes precedence)
• Improved the non 200 error message to include the statusCode

Features

• Can now use inline URL auth

Bug Fixes

See above

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

[KhafraDev]

can you run npm run lint and fix the errors?

[codecov-commenter]

Codecov Report

Attention: 60 lines in your changes are missing coverage. Please review.

Comparison is base (e39a632) 85.54% compared to head (26ff0df) 85.62%.
Report is 115 commits behind head on main.

Files	Patch %	Lines
lib/handler/RetryHandler.js	73.04%	31 Missing ⚠️
lib/fetch/index.js	64.51%	11 Missing ⚠️
lib/fetch/headers.js	88.63%	5 Missing ⚠️
lib/api/readable.js	80.00%	4 Missing ⚠️
lib/client.js	92.10%	3 Missing ⚠️
lib/core/util.js	90.47%	2 Missing ⚠️
lib/fetch/request.js	90.00%	2 Missing ⚠️
lib/core/request.js	97.56%	1 Missing ⚠️
lib/proxy-agent.js	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2473      +/-   ##
==========================================
+ Coverage   85.54%   85.62%   +0.07%     
==========================================
  Files          76       77       +1     
  Lines        6858     7059     +201     
==========================================
+ Hits         5867     6044     +177     
- Misses        991     1015      +24     

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