[Feature]Support managing existing Apache Iceberg tables with Fluss

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Motivation

Description:
We are evaluating Apache Fluss and would like to use it on top of some existing Apache Iceberg tables.
We attempted to create a Fluss PK table pointing to the same iceberg table, but the table creation fails when the underlying Iceberg table already exists.

Seems Fluss currently expects to create and manage the table from scratch, and does not support, Attaching to an existing Iceberg table

### Solution

it would be great if fluss can manage the existing datalake tables, like iceberg

### Anything else?

_No response_

### Willingness to contribute

- [x] I'm willing to submit a PR!

The issue names no files, tests, or entry points. Start by reproducing the failure when creating a Fluss PK table over an existing Iceberg table, then trace the table-creation path and its Iceberg integration. Done means Fluss can manage an existing Iceberg table rather than requiring creation from scratch, with coverage for that case.

Written by the indexing model from the issue text.