Add proxy protocol v2 client-side support

[defanator]

Proposed changes

This PR introduces support for proxy protocol v2 which is available in nginx starting from versions 1.13.11 (foss) and R16 (NGINX Plus).

Primary use case is running nginx-prometheus-exporter alongside with nginx or NGINX Plus sitting behind other load balancers like AWS NLB or Google's PNLB with proxy protocol enabled. Sometimes creating dedicated listeners just for stub_status or API could be undesired, and with the proposed changes the exporter will be able to reuse existing listeners without extra effort.

Changes were tested with the following combinations of listeners:

1. IPv4 with proxy_protocol, IPv4 without proxy_protocol.
2. IPv6 with proxy_protocol, IPv6 without proxy_protocol.
3. Unix socket with proxy_protocol, unix socket without proxy_protocol (not that common scenario, but still supported by standard).

Implementation used: https://github.com/pires/go-proxyproto

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING guide
• I have proven my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have ensured the README is up to date
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch on my own fork

[github-actions]

✅ All required contributors have signed the F5 CLA for this PR. Thank you!
_{Posted by the CLA Assistant Lite bot.}

[defanator]

I have hereby read the F5 CLA and agree to its terms

[defanator]

For the reference, here's the linter output from a platform I've been testing on:

nginx-prometheus-exporter % make lint
go run github.com/golangci/golangci-lint/cmd/[email protected] run --fix

nginx-prometheus-exporter % go version
go version go1.24.0 darwin/amd64

nginx-prometheus-exporter % sw_vers 
ProductName:            macOS
ProductVersion:         15.3
BuildVersion:           24D60