[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

從 Java Datasets API 現有的下推投影支援開始，接著檢查 ScanOptions 和 Expression 物件在 Java 與 C++ 元件中的使用方式。當 Java 篩選述詞可以透過 ScanOptions 傳遞，以實現下推篩選，同時不會造成難以持續的維護負擔時，工作即告完成。

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