feat: (2 of 2) Upstream module example

[f5yacobucci]

Upstream example

• derived from github.com/gabihodoroaga/nginx-upstream-module and NGINX ngx_http_upstream_keepalive_module.c

Simply replaces peer functions with passthrough versions, i.e., proxies traffic through this module to originally configured upstream functions.

Useful as a guide and example, but does not add load balancing algorithms.

Fixes #30 (replaces PR #35)

Checklist

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• [X ] I have written my commit messages in the Conventional Commits format.
• [X ] I have read the CONTRIBUTING doc
• [X ] I have added tests (when possible) that prove my fix is effective or that my feature works
• [X ] I have checked that all unit tests pass after adding my changes
• [X ] I have updated necessary documentation
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• [X ] I will ensure my PR is targeting the main branch and pulling from my branch from my own fork