Tests: proxy_header_timeout directive

[vinaykumar-1591]

Proposed changes

Tests to cover supporting prxy_header_timeout directive which aims to support timeouts.backendRequest defined from specification The gateway API specification.

companion source PR nginx/nginx#1738

Checklist

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• I have read the contributing guidelines.
• I have signed the F5 Contributor License Agreement (CLA).
• If applicable, I have added tests that prove my fix is effective or that my feature works.
• If applicable, I have checked that any relevant tests pass after adding my changes.
• I have updated any relevant documentation (README.md and/or CHANGELOG.md).

[route443]

Hi @vinaykumar-1591 ,

I've rewritten your commit. There are 5 main points:

1. An nginx version check is needed. Specifically, this set of tests must be successfully skipped on versions older than 1.31.6. The simplest way to do this is via a separate .t. plus, this is established practice for the new directive.
2. There is no check for the send phase, exactly what @arut explicitly requested in the PR.
3. There is no check for proxy_next_upstream classification. This is what allowed the ssl issue to slip through.
4. The proxy.t is already heavily loaded and takes a long time to exec. this allows for parallel run (prove -j).
5. Added you as co-author

[route443]

The sandbox CI passes without issues against the ba02005. LGTM