[Feature] Add practical field aggregators available in Java Paimon

### Search before asking

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

### Motivation

Complete the Paimon C++ SDK with practical field aggregators that are already supported by Java Paimon.

### Solution

Add support for the following field aggregators:

- `collect`
- `merge_map`
- `nested_update`
- `rbm64`
- `hll_sketch`
- `theta_sketch`

### Anything else?

_No response_

### Are you willing to submit a PR?

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

Start by locating the existing C++ field-aggregator entry points and the corresponding Java Paimon implementations referenced in the issue. Compare the behavior of collect, merge_map, nested_update, rbm64, hll_sketch, and theta_sketch, then run the relevant aggregator tests; done means all six are supported consistently.

Written by the indexing model from the issue text.