[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

Start with the Java Datasets API's existing push-down projection support, then inspect how ScanOptions and the Expression object are used across the Java and C++ components. The work is complete when Java filter predicates can be passed through ScanOptions for push-down filtering without creating an unsustainable maintenance burden.

Written by the indexing model from the issue text.