[spark] use CamelCase in spark configs

### Search before asking

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

### Description

as per title

### Willingness to contribute

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

Start by searching the repository for Spark configuration definitions and usages, then determine which configuration names are covered by the request. Confirm the expected CamelCase convention and identify related tests before making changes; done means the applicable Spark configs consistently use CamelCase and the relevant tests pass.

Written by the indexing model from the issue text.