feat(contrib/haproxy): HAProxy AAP support

[e-n-0]

What does this PR do?

This PR implements an HAProxy SPOE messages handler that can be used to create full featured AAP HAProxy agent (coming part of #3913).

This PR uses the message_processor package implemented from #3860 and implements its own HAProxy message processor.

This implementation use the library github.com/negasus/haproxy-spoe-go for the communication between haproxy and the agent (implementing the SPOP).

Motivation

New proxy integration supported for AAP: HAProxy

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running ./scripts/lint.sh locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-09-24 09:45:47

Comparing candidate commit 39c6213 in PR branch flavien/contrib/haproxy with baseline commit a7d75a7 in branch main.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 2 metrics, 0 unstable metrics.

scenario:BenchmarkSetTagStringer-24

• 🟥 execution_time [+2.471ns; +8.529ns] or [+2.026%; +6.993%]

[datadog-official]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 39c6213 | Docs | Was this helpful? Give us feedback!}

[e-n-0]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. You're on a roll.

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