KIP-740 — Clean up public API in TaskId

Accepted Kafka 3.0 Streams

Changes `TaskMetadata.taskId()` to return a `TaskId` object instead of its `String` encoding so callers receive a strongly-typed value with direct access to sub-topology and partition fields. The `String` representation required callers to parse the encoding themselves, and made it impossible to add new `TaskId` fields without breaking existing parsing code.

Details