Introduce generic secondary index metadata foundation

### 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!

Start by reviewing the schema and table metadata layer, then identify how a generic secondary-index model can support future index types and validation. Done means establishing the metadata and validation baseline; runtime lifecycle, replication, and the GSI lookup path are explicitly out of scope.

Written by the indexing model from the issue text.