[C++] ReadRel is translated to a source node that emits unexpected fields

[asfimport]

Currently, a Substrait plan with a RelRoot containing a ReadRel will contain extra, unexpected fields, namely __fragment_index et. al. Right now they are always included by default. There are a few things to be done:

• ReadRel's base_schema could be converted into a ScanOptions.dataset_schema to limit the fields read. (Also see ARROW-15585, these fields should be used for pushdown projection)
• The scanner always adds these extra fields - maybe it should be opt-in instead
• There's no way to manually insert a Project to "fix" things because as implemented, it can only add new columns

Reporter: David Li / @lidavidm

Related issues:

• [C++] Add support for Substrait pushdown projection (relates to)

_{Note: This issue was originally created as ARROW-17229. Please see the migration documentation for further details.}

[github-actions]

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