Use the backfill pulsar admin ext for e2e tests

[aymkhalil]

The patch updates the Cassandra CDC backfill CLI to use the Pulsar Admin CLI Extension for e2e testing. The motivation:

1. The "extension" is the recommend way to run backfilling.
2. It already exercise the code path that was previously tested if someone where to run the jar directly with the exception of a thin wrapper for the CLI entry point.

If necessary, we could enable tests on both the JAR and the NAR artifacts but this seems like an overkill for the time being.

Update: I'll fall back to testing with the JAR when CLI ext is not enabled.