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What does this PR do?

This PR changes pipeline tracing logic.

When org.datadog.jenkins.plugins.datadog.listeners.DatadogBuildListener#onStarted listener is invoked for a pipeline, the accurate start time for a pipeline is not known yet, because until the first pipeline step starts its execution the pipeline is considered queued.
The backend relies on pipeline start time being consistent (due to some EVP constraints).
Therefore we have to defer sending the first tracing event for pipelines until their accurate start time is known, which is when the "Start of Pipeline" FlowNode is encountered.

Description of the Change

Alternate Designs

Possible Drawbacks

Verification Process

Additional Notes

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Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
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