fix(ddtrace/tracer): [draft] evolving v1 trace protocol

[darccio]

What does this PR do?

Sets a different direction for #3947 by getting payloadV1 closer to the RFC representation.

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!

[datadog-official]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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[pr-commenter]

Benchmarks

Benchmark execution time: 2025-10-06 13:21:18

Comparing candidate commit 5078e30 in PR branch dario.castane/v1-trace-protocol with baseline commit 60165d3 in branch hannahkm/implement-v1-serialization.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 24 metrics, 0 unstable metrics.