Support ControlledShutdown for TabletServer

[swuferhong]

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Motivation

Currently, Fluss's TabletServer does not perform any operations when shutdown. Only after the CoordinatorServer detects the server's offline status will it initiate the migration of Bucket Leaders. This approach increases cluster instability during machine scale-in/scale-out events. Therefore, this issue proposes a ControlledShutdown mechanism, where the TabletServer actively notifies the CoordinatorServer to migrate Bucket Leaders before shutdown.

Solution

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Anything else?

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Willingness to contribute

• I'm willing to submit a PR!