Add Datadog code coverage upload

[ManuelPalenzuelaDD]

What does this PR do?

We're migrating Datadog repositories from Codecov to Datadog Code Coverage for tracking test coverage. This PR is the first step: it adds a Datadog coverage upload alongside the existing Codecov upload so we can run both systems in parallel and verify parity before switching over.

Changes

• Added upload-coverage job to master.yml that downloads coverage artifacts and uploads to both Codecov and Datadog (matching the pattern already used in integrations-core's master.yml)
• Added code-coverage.datadog.yml with global carryforward enabled (matching .codecov.yml carryforward settings for 78+ flags)
• The PR workflow already uploads to Datadog via integrations-core's pr-test.yml reusable workflow
• The existing Codecov uploads are unchanged — nothing is removed or modified
• Coverage format: Cobertura XML (generated by test-target.yml as coverage.xml per integration)

Why are we doing this?

As part of a company-wide effort, we're consolidating code coverage reporting into Datadog's own Code Coverage product. This gives us:

• Coverage data integrated directly into Datadog CI Visibility
• PR gates and coverage checks natively in Datadog
• No dependency on a third-party service (Codecov) for coverage reporting

Validation

Pending — no tests ran on this config-only PR. Coverage comparison will be possible after a CI run with actual test execution.

Next steps (not in this PR)

Once this PR is merged and we've confirmed Datadog coverage is stable over several commits:

1. Remove the Codecov upload steps and CODECOV_TOKEN secret
2. Remove .codecov.yml
3. Optionally configure PR gates in code-coverage.datadog.yml

No action needed from reviewers beyond normal review

This is a low-risk, additive change. The new upload steps run independently of the existing CI pipeline and cannot cause test failures.

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