[RRIO] [Throttle] stub the dequeue transform

[damondouglas]

This PR addresses #28932 by creating a stub of a PTransform that dequeues elements at a fixed rate.

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[damondouglas]

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[chamikaramj]

Accidentally merged ?

Please don't merge PRs without a peer review: https://github.com/apache/beam/blob/master/CONTRIBUTING.md#review-process-and-releases

[damondouglas]

Accidentally merged ?

Please don't merge PRs without a peer review: https://github.com/apache/beam/blob/master/CONTRIBUTING.md#review-process-and-releases

Oh no. I did accidentally seeing the green check box. There were a few that were approved at once. Should I revert?

[chamikaramj]

LGTM.

[chamikaramj]

It's fine. I LGed posthumously :)

[damondouglas]

It's fine. I LGed posthumously :)

Thank you so much for all your help :-)