♻️ [RUMF-1533] extract the Flush logic into a reusable component

[BenoitZugmeyer]

Motivation

The flush logic will become a bit more complex in the short term. Let's extract the flush logic from Batch to separate concerns. In a future PR, the newly introduced flush controller will also be used for flushing segments.

Changes

• Implement a flush controller to handle all flush use cases
• Use it in Batch
• Use it for customer data telemetry

Testing

• Local
• Staging
• Unit
• End to end

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I have gone over the contributing documentation.

[amortemousque]

❓ question: ‏Why do you need this?

[BenoitZugmeyer]

I need to call reset to reset the number of calls after the test setup, so I can use expect().toHaveBeenCalledOnceWith(...) (only the last call is relevent to this test case)