Currently it's impossible to use inner classes for converters, also usually instead of get `is`/`has` could be used for booleans

### Search before asking

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


### Motivation

Put in in task name

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

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