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GH-35579: [C++] Support non-named FieldRefs in Parquet scanner (#35798)



### Rationale for this change

When setting projections/filters for the file system scanner, the Parquet implementation requires that all materialized `FieldRef`s be position-independent (containing only names). However, it may be useful to support index-based field lookups as well - assuming the dataset schema is known.

### What changes are included in this PR?

Adds a translation step for field refs prior to looking them up in the fragment schema. A known dataset schema is required to do this reliably, however (since the fragment schema may be a sub/superset of the dataset schema) - so in the absence of one, we fall back to the existing behavior.

### Are these changes tested?

Yes (tests are included)

### Are there any user-facing changes?

Yes

* Closes: #35579

Authored-by: benibus <bpharks@gmx.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>