[RRIO] [Call] Implement PTransform without adaptive throttling

[damondouglas]

This PR addresses #28711 by implementing the Call PTransform that executes user custom code.

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[chamikaramj]

Thanks.

[chamikaramj]

Thanks. LGTM.

[damondouglas]

Thanks. LGTM.

Thank you @chamikaramj for your help and insight with this. It still shows you are requesting changes and wanted to make sure all was well before Squash/Merge.

[chamikaramj]

Thanks. Please self merge when tests pass.