[C++] Determine how to represent dictionary encoding in Substrait

[asfimport]

Substrait does not appear to include dictionary encoded types in the spec at the moment. We need to figure out how we want to represent this in substrait (e.g. custom type variations? new custom types?), potentially create YAML files, and potentially create Substrait issues / discussion to ensure we are aligned on our approach.

Reporter: Weston Pace / @westonpace

_{Note: This issue was originally created as ARROW-15536. Please see the migration documentation for further details.}

[github-actions]

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