[Feature] Add distinct parameter support to FieldListaggAgg operator

### Search before asking

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

### Motivation

Users often store complex data such as `user1, user2, user3` as simple string rather then complex date type (map,array).
When aggregating these fields, there is currently no support for deduplication `set.add`.

### Solution

Add a `distinct` option support to FieldListAgg?

### Anything else?

This approach works fine for `user1,user2,user3`, but it's insufficient for kv-string deduplication `k1=v1;k2=v2`. I'm not sure whether to handle this with string manipulation or a map-merge aggregation approach due to performance concerns..

### Are you willing to submit a PR?

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

The entry point named by the issue is the FieldListaggAgg operator; start by locating its implementation and related aggregation tests. Determine how a distinct option should apply to comma-separated values and clarify whether key-value string deduplication is in scope. Done means the supported distinct behavior is implemented and covered by tests.

Written by the indexing model from the issue text.