Substrait SetRel

I would like to use the SetRel operator to union two datasets. I got the error: `NotImplemented: conversion to arrow::compute::Declaration from Substrait relation set{...}`. According to the `relation_internal.cc`, it looks like we have not implemented this operator. Is there any way to address this problem or any plan to implement the operator in the future?

The Substrait plan is as the following:
[myplan.txt](https://github.com/apache/arrow/files/9418427/myplan.txt)

Start by reading relation_internal.cc and the attached myplan.txt to understand how the Substrait SetRel plan reaches conversion to arrow::compute::Declaration. Trace the existing relation conversion paths and determine what is needed for the reported union of two datasets. Done means the supplied plan converts successfully without the current NotImplemented error.

Written by the indexing model from the issue text.