KIP-131 — Add access to OffsetStorageReader from SourceConnector

Accepted Kafka 2.6 Connect

Exposes an `OffsetStorageReader` to `SourceConnector` (not just `SourceTask`) so connector implementations can read committed source offsets in the `Connector` class itself. Without this, a `SourceConnector` background thread that scans for new source data (e.g. new files) cannot determine which files have already been fully processed, preventing task reconfiguration based on offset state.

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