Add support for aggregate functions like `list_agg` return complex data types

### Search before asking

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

### Description

Add support for `list_agg` aggregate functions that returns complex data types.

### Willingness to contribute

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

The issue names no files, entry points, or tests. Start by locating the existing list_agg aggregate implementation and the tests for aggregate return types, then trace how complex data types are represented. Done means list_agg can return complex data types with coverage for the supported cases.

Written by the indexing model from the issue text.