KIP-744 — Migrate TaskMetadata and ThreadMetadata to an interface with internal implementation

Accepted Kafka 3.0 Streams

Converts `TaskMetadata` and `ThreadMetadata` from concrete classes to interfaces with internal implementations, separating the public API contract from the implementation details. Users should never instantiate these metadata objects directly—making them concrete classes with public constructors prevents the Streams team from safely evolving the internal representation.

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