bug: possible data loss for MQTT source

[tabVersion]

Describe the bug

Although we record the pkid (as offset) of each message, we cannot subscribe to the topic from spec offset.
And we cannot do the manual ACK on our side, because the message's ACK is tied to client. We cannot do ack from a different client (in WaitCheckpointTask).

Error message/log

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To Reproduce

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Expected behavior

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How did you deploy RisingWave?

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The version of RisingWave

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Additional context

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