When working with strict type checking in typescript, the callbacks dr_cb, rebalance_cb and offset_commit_cb are not sufficiently typed, and even the examples given in the README are throwing type errors.

This PR defines strict types for these callbacks.

I have done my best to back track the types from the EventListenerMap for rebalance, offset.commit and delivery-report

Caveat 1: I have not been able to build the project locally, so these changes are made based on the installed package from npm inside a project that consumes node-rdkafka

Caveat 2:. In my project, I am currently only using rebalance_cb, so the others have been harder to check.