PYTHON-5562: Investigate default values for withTransaction backoff parameters

[sleepyStick]

PYTHON-5562

Summary

_BACKOFF_INITIAL, _BACKOFF_MAX, and the growth rate have now been experimented with using the previous with_transaction benchmark and values are chosen based on what experimentally did the best.

Changes in this PR

• added a test for the backoff in with_transaction
• changed _BACKOFF_INITIAL, _BACKOFF_MAX, and the growth rate

Testing Plan

prose test

Screenshots (optional)

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is the intention of the code captured in relevant tests?
• If there are new TODOs, has a related JIRA ticket been created?

Checklist for Reviewer {@primary_reviewer}

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Have you checked for spelling & grammar errors?
• Is all relevant documentation (README or docstring) updated?

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