Support Tiering Service registration with Fluss Cluster

[leonardBang]

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Description

Tiering Service registers with Fluss Cluster via heartbeat RPC containing current task status (managed tables + capacity for new tasks). Initial heartbeat (equivalent to registration) defaults to requesting tasks (empty managed tables + available capacity). Subsequent heartbeats determine task requests based on sync progress.

Willingness to contribute

• I'm willing to submit a PR!

[luoyuxia]

Completed via #874