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GH-36214: [C++] Specify `FieldPath::Hash` as template parameter where possible (#36222)



### Rationale for this change

Internal specializations of `std::hash<FieldPath>` have caused multiple-definition errors with unity builds.

### What changes are included in this PR?

To avoid exposing a global specialization of `std::hash` (which must be declared in the `std` namespace), this specifies the `FieldPath::Hash` helper as a template parameter to hashable containers instead.

### Are these changes tested?

Yes (covered by existing tests)

### Are there any user-facing changes?

No

* Closes: #36214

Authored-by: benibus <bpharks@gmx.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>