[Improvement] Introducing APIs to show the progress of scheme change.

### What would you like to be improved?

As the schema changes in some catalogs, such as Doris, TiDB is asynchronous and it takes a lot of time for operations like adding an index, so I wonder if we need to add an extra API to show the process of the schema change.
With it, users can easily see when the changes have been completed and then proceed with the next steps.

@zhoukangcn @jerryshao

### How should we improve?

_No response_

Review the existing catalog schema-change operations and API entry points, especially the asynchronous paths for Doris and TiDB. Compare how current APIs expose operation state and identify the completion signal and response shape needed; done means an agreed API contract with tests covering progress and completion.

Written by the indexing model from the issue text.