WIP: implement DD_TRACE_INTEGRATION_TAGS + contrib/net/http support

[rarguelloF]

What does this PR do?

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.

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[datadog-datadog-prod-us1]

Datadog Report

Branch report: rarguelloF/poc-env-integration-config
Commit report: d7bb46b
Test service: dd-trace-go

❌ 1 Failed (0 Known Flaky), 5222 Passed, 73 Skipped, 1m 16.41s Total Time

❌ Failed Tests (1)

• TestServeMuxNoRace - gopkg.in/DataDog/dd-trace-go.v1/contrib/net/http - Details

  Expand for error

   Failed
   
   === RUN   TestServeMuxNoRace
   component=net/http;http.request.method=GET;server.address=;server.port=;span.kind=server;url.path=/
   component=net/http;http.request.method=GET;server.address=;server.port=;span.kind=server;url.path=/
   ==================
   Read at 0x00c0003135f0 by goroutine 78:
     runtime.mapaccess2_faststr()
         /opt/hostedtoolcache/go/1.22.12/x64/src/runtime/map_faststr.go:108 +0x0
     gopkg.in/DataDog/dd-trace-go.v1/internal.(*IntegrationTags).Get()
   ...