Show all Flink-compatible Kafka clusters (as databases) if no pool is passed to quickpick

The pool is currently required for the filter predicate: https://github.com/confluentinc/vscode/blob/0855fd5dd59ee2fa4285cec94dca5a65cd0bfa7f/src/quickpicks/kafkaClusters.ts#L178

But by the time we have all of the "coarse" resources loaded, we should be able to determine what Flink-compatible Kafka clusters exist by matching cloud provider/region combinations:
- extract all provider/region combos from compute pools
- update the filter predicate so the Kafka cluster has to match a provider/region combo