[I] Introduce generic secondary index metadata foundation [fluss]

["platinumhamburg (via GitHub)" <gi...@apache.org>]

platinumhamburg opened a new issue, #3155:
URL: https://github.com/apache/fluss/issues/3155

   ### Search before asking
   
   - [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.
   
   
   ### Description
   
   The goal of this task is to establish a metadata model that supports secondary indexes in the current stage, while also leaving enough extensibility in the schema/table metadata layer for future index types without requiring repeated incompatible metadata changes.
   
   This task focuses on the metadata and validation baseline only. It does not yet implement the full runtime lifecycle, replication, or lookup path of GSI.
   
   ### Willingness to contribute
   
   - [x] I'm willing to submit a PR!


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