KIP-321 — Update TopologyDescription to better represent Source and Sink Nodes

Accepted Kafka 2.1 Streams

Updates `TopologyDescription` to expose `SourceNode` and `SinkNode` as typed objects with structured accessors (topics, pattern, etc.) instead of returning string representations only. Users who want to perform runtime introspection on topology source/sink nodes are forced to parse `toString()` output, which is brittle and not part of the stable API contract.

Details