[Feature] Support to delete the row in aggregation merge engine

### Search before asking

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

### Motivation

Currently, the aggregation merge engine only produce `INSERT` row. However, if all the input of a key has been retract, it should produce a DELETE row.

### Solution

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

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

Start by locating the aggregation merge engine and reading how it handles retracted inputs and currently produces INSERT rows. The change is complete when a key whose inputs have all been retracted produces a DELETE row, with coverage for that behavior.

Written by the indexing model from the issue text.