[client][server] Support adding aggregation columns

[wzx140]

Summary

• Support Java API TableChange.addColumn interfaces to pass aggregation functions when altering table schema.
• Carry the aggregation function through ALTER TABLE ADD COLUMN RPC so the server preserves it in the new schema.
• Validate aggregation functions on schema alter for aggregation merge engine tables and reject them on non-aggregation tables.

[Copilot]

Pull request overview

Note

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Adds end-to-end support for specifying aggregation functions when adding columns via ALTER TABLE ADD COLUMN, including RPC transport and server-side validation for aggregation vs non-aggregation merge engine tables.

Changes:

• Extend TableChange.addColumn to optionally carry an AggFunction.
• Add RPC support to serialize/deserialize aggregation functions for ADD COLUMN.
• Enforce validation rules for aggregation functions on schema alter and table creation.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
fluss-server/src/main/java/org/apache/fluss/server/utils/TableDescriptorValidation.java	Adds validation to reject aggregation functions for non-aggregation merge engines; exposes validation helpers for reuse.
fluss-server/src/main/java/org/apache/fluss/server/utils/ServerRpcMessageUtils.java	Deserializes aggregation function from PbAddColumn to TableChange.AddColumn.
fluss-server/src/main/java/org/apache/fluss/server/coordinator/SchemaUpdate.java	Applies AggFunction when building the updated schema for added columns.
fluss-server/src/main/java/org/apache/fluss/server/coordinator/MetadataManager.java	Validates aggregation function usage during ALTER TABLE schema updates based on merge engine.
fluss-rpc/src/main/proto/FlussApi.proto	Adds serialized_agg_function field to PbAddColumn.
fluss-common/src/main/java/org/apache/fluss/metadata/TableChange.java	Adds new addColumn overload carrying AggFunction and stores it on AddColumn.
fluss-client/src/test/java/org/apache/fluss/client/admin/FlussAdminITCase.java	Adds integration tests for altering aggregation and non-aggregation tables with aggregation functions.
fluss-client/src/main/java/org/apache/fluss/client/utils/ClientRpcMessageUtils.java	Serializes aggregation function into PbAddColumn.

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[wzx140]

@luoyuxia Could you please help review this one?