Implement diagnostic flare

[nikita-tkachenko-datadog]

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What does this PR do?

Implements diagnostic flare functionality.
A new Datadog page is added to Manage Jenkins -> Troubleshooting.
In this page there is a button to generate and download a diagnostic flare archive.
The archive contains data that is useful for investigating issues with the plugin.

Description of the Change

Alternate Designs

Possible Drawbacks

Verification Process

Additional Notes

Release Notes

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.
• PR should not have do-not-merge/ label attached.
• If Applicable, issue must have kind/ and severity/ labels attached at least.