[RRIO] [Throttle] Stub throttle without external resource

[damondouglas]

This PR addresses #28930 by stubbing a transform that aims to throttle a PCollection without using an external resource.

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[damondouglas]

Run Java PreCommit

[github-actions]

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[damondouglas]

assign set of reviewers

[github-actions]

Reviewers are already assigned to this PR: @kennknowles @ahmedabu98

[ahmedabu98]

Pretty simple, LGTM

[ahmedabu98]

sorry, meant to approve