[v2] run go generate and add ci check

[rarguelloF]

What does this PR do?

• Run go generate ./...
• Remove duplicated SpanLink struct in the ddtrace package.
• Check generated files are up to date in CI.
• Remove duplicated make_responsewriter.go file and fix the used one.
• Run tools/fixmodules which ensures all modules in the repo are go mod tidy'ed and have all the correct replace for local imports. Also add a CI check running the tool and checking it does not generate changes.

Motivation

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.
• 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.
• For internal contributors, a matching PR should be created to the v2-dev branch and reviewed by @DataDog/apm-go.

Unsure? Have a question? Request a review!

[datadog-datadog-prod-us1]

Datadog Report

Branch report: rarguelloF/gen-span-msgp
Commit report: 8ffc238
Test service: dd-trace-go

✅ 0 Failed, 68 Passed, 0 Skipped, 3m 0.6s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-03-26 14:16:44

Comparing candidate commit ff3c1af in PR branch rarguelloF/gen-span-msgp with baseline commit 57473e2 in branch v2-dev.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 55 metrics, 1 unstable metrics.