[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

Java Datasets API に既存する push-down projection のサポートから始め、その後、Java および C++ コンポーネント全体で ScanOptions と Expression オブジェクトがどのように使用されているかを調査します。Java のフィルタ述語を ScanOptions 経由で push-down filtering に渡せるようになり、持続不可能な保守負担が生じなければ、作業は完了です。

索引モデルが issue の本文から書いたものです。