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What does this PR do?

Ensures that (arguably) malformed pipelines that seem to be neither scripted nor declarative are displayed in Datadog - not only once finished, but also while running.

The way running pipelines work is that the plugin submits a pipeline to the backend once it has started, and later submits the same pipeline once it has finished - the backend updates pipeline record and the pipeline status changes in the UI.
The caveat with the running pipelines is that Datadog backend requires that a pipeline start time never changes: there are some technical limitations related to how pipeline data is stored, due to which the start time needs to be constant.
The problem is that the true start time of a pipeline is only known while the pipeline has left the build queue (while the pipeline is in the queue it has not started running yet), which for some pipelines happens after pipeline listeners have been invoked (this concerns the build listener, the graph listener, the step listener, the SCM checkout listener - all of these listeners may fire before pipeline leaves the build queue).

To ensure that the start time does not change the plugin does not submit pipeline data to the backend if the timestamp of that pipeline leaving the build queue is not known yet.
The problem is that some pipelines are executed without being added to the build queue.
Here is an example of such pipeline (missing the pipeline {} and the node {} top-level blocks):

echo "This is a step that doesn't belong to any stage"

stage("Bootstrap") {
    echo "And this is a regular step"
}

echo "This is another step that doesn't belong to any stage"

The workaround introduced in this PR is: when the DatadogStepListener detects that a pipeline step has started executing, it checks the queue time data - if the data was not filled and the pipeline is executing already, it means that the pipeline missed the build queue. For such pipelines the queue time is set to 0.

Description of the Change

Alternate Designs

Possible Drawbacks

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Additional Notes

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• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
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