[Java] Push-down filtering in Java

### Push-down filtering (Java) questions

Hey all,

I'm exploring adding push-down filtering to the Java Datasets API, as it currently only supports push-down projection.
Just had a couple of questions before I start this work:

- Is this planned on any current roadmap? I'd like to avoid duplicating work that's already begun
- Does anyone have any smart ideas as to how to pass the filter predicate into ScanOptions? As far as I can tell, the only complication to make this happen would be figuring out some way to call into the Expression object from Java without creating future maintenance effort.

Thanks!

### Component(s)

C++, Java

Comienza con la compatibilidad existente con la proyección push-down de la Java Datasets API y, después, inspecciona cómo se usan ScanOptions y el objeto Expression en los componentes de Java y C++. El trabajo estará completo cuando los predicados de filtro de Java puedan pasarse a través de ScanOptions para el filtrado push-down sin crear una carga de mantenimiento insostenible.

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