client: add retry policy param to start_workflow

[Herklos]

What was changed

Retry policy attribute has been added to client start_workflow method to allow the Rust client to specify a retry policy when starting a workflow.

Why?

It's a useful param for some use cases and it was not available on Rust client while it is available for client of other languages (like go, typescript or python).

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[Sushisource]

Thanks for this!