[Rust] [DataFusion] Subquery support

[Andy Grove <an...@gmail.com>]

I am mostly working full-time on DataFusion for the immediate future and
would like to take the lead in coordinating the effort to add support for
subqueries, working with other contributors to implement the many features
that are required, including planning, optimizations, and execution.

I have started a design document [1] and I have created an epic issue in
GitHub [2] to track the work since there are already many issues and
discussions related to this.

If you are interested in this work then please review the design document
and add any feedback that you have.

Thanks,

Andy.

[1]
https://docs.google.com/document/d/1j5vHyva-T_5l3POnHSS0-r5TOp86sEkfSUcRLqJZaK4/edit?usp=sharing
[2] https://github.com/apache/arrow-datafusion/issues/2248