[Feature] support user defined FieldAggregator

### Search before asking

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

### Motivation

We are planning to migrate our business from Hudi to Paimon. Its partial field update mechanism greatly improves multi-stream merging jobs. Unfortunately, we use a custom payload for field merging, and Paimon does not have a similar extension mechanism.

### Solution

We hope that later versions could support custom aggregate function.

### Anything else?

_No response_

### Are you willing to submit a PR?

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

Start by reading Paimon's partial field update and FieldAggregator implementation and trace how field merging is selected. Define the extension points and compatibility requirements for a user-defined aggregate function, then add coverage showing a custom merger is invoked during partial updates.

Written by the indexing model from the issue text.