[C++] allow unify schema to coalesce int64 and float64

[asfimport]

Need to make a reprex for this still, but one example where it might not work is: dataset with two partitions, one has int64 and one has float64, then unify schemas

Reporter: Jonathan Keane / @jonkeane

Related issues:

• [C++] unify_schemas can't handle int64 + double, affects CSV dataset (is duplicated by)

_{Note: This issue was originally created as ARROW-14695. Please see the migration documentation for further details.}