feat: delete Kafka consumer group when source backfill is finished

[xxchan]

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Checklist

• I have written necessary rustdoc comments.
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Release note

Now Kafka consumer groups created by RisingWave Kafka source during backfill will be deleted when the backfill process is finished.

[xxchan]

• feat: delete Kafka consumer group when source backfill is finished #20077 👈 (View in Graphite)
• feat: delete Kafka consumer group on drop #20065
• refactor(meta): splits source_manager into smaller mods #20071
• main

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Merge activity

• Jan 17, 9:15 AM GMT+8: Graphite rebased this pull request after merging its parent, because this pull request is set to merge when ready.