Use c.git.repository_url and c.git.commit_sha if it is defined instead of env vars

[vpellan]

What does this PR do?

This PR replace the direct access to DD_GIT_REPOSITORY_URL and DD_GIT_COMMIT_SHA to access to the config DSL. There is a fallback mechanism if the option is not defined on dd-trace-rb (for older versions).

Motivation

We're trying to move as much env vars as possible to the config DSL, so that they can be configured programmatically, but also through stable config. These two configs are common to both dd-trace-rb and datadog-ci-rb.

Additional Notes

Depends on DataDog/dd-trace-rb#5559

How to test the change?

[datadog-prod-us1-5]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 78.38%
• Overall Coverage: 97.04% (-0.03%)

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