[Client] Add the primary-key table bulk-load client SDK

[platinumhamburg]

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Description

Provide a public client SDK over the server-side bulk-load protocol.

The SDK should hide RPC messages, manifest assembly, snapshot file handling,
and Commit retry details while supporting both single-process builders and
distributed execution engines.

Scope

• Expose BulkLoad through the Fluss connection.
• Add a client for Begin, GetInProgress, Commit, and Abort.
• Return an immutable and serializable build context from Begin.
• Add openBuildSession as the entry point for local build resources.
• Create bucket writers through the build session.
• Route and validate rows against the frozen target metadata.
• Deduplicate primary-key rows locally, with the last row for a key winning.
• Build ordinary Fluss KV snapshot files for each bucket.
• Support an explicit log end offset when required by the caller.
• Assemble and publish the complete manifest internally.
• Retry Commit within a caller-provided await budget when the outcome is
  retriable or unknown.
• Close all writers still owned by a build session when the session is closed.
• Keep credentials and process-local resources out of the serializable build
  context.

Willingness to contribute

• I'm willing to submit a PR!