Add custom sleep parameter to async_bulk and async_streaming_bulk

[girolamo-giordano]

Add custom sleep parameter to async_bulk and async_streaming_bulk allowing users to define their custom sleep action on async_bulk and async_streaming_bulk calls
Closes #2484

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[pquentin]

Hello, and thank you! We'll also need to add this to the sync version and add tests in test_helpers.

cla/check

[miguelgrinberg]

@pquentin Do you think we can close this one? I assume the purpose was to be able to use trio's sleep?