KIP-661 — Expose task configurations in Connect REST API

Accepted Kafka 2.8 Connect

Adds a `GET /connectors/{connector}/tasks/{task}/config` REST endpoint to expose the computed task configuration as applied by the connector, not just the raw connector config. Connectors like `MirrorSourceConnector` or `JdbcSourceConnector` transform the connector config before distributing it to tasks; the resulting per-task config is not currently retrievable via the REST API.

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