feat(client): add transaction isolation level

[jonathanblade]

Change Summary

Add support for interactive transactions with an isolation level.

Closes https://github.com/RobertCraigie/prisma-client-py/issues/463.

Checklist

• [x] Unit tests for the changes exist
• [x] Tests pass without significant drop in coverage
• [x] Documentation reflects changes where applicable
• [x] Test snapshots have been updated if applicable

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