[Aggregation] Add collect/merge_map aggregation support

### Search before asking

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

### Description

- Add COLLECT and MERGE_MAP aggregation functions with distinct handling and map/array merge logic.
- Align aggregation output encoding with KV row format.
- Update docs and extend tests for new aggregation behavior.

### Willingness to contribute

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

Locate the existing aggregation implementation, output encoding, documentation, and aggregation tests first. Implement COLLECT and MERGE_MAP with distinct handling, map/array merge behavior, and KV row-compatible encoding; update the docs and extend tests to demonstrate the new behavior.

Written by the indexing model from the issue text.