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GH-38874: [C++][Parquet] Minor: making parquet TypedComparator operation as const method (#38875)



### Rationale for this change

`parquet::TypedComparator<DType>` is not const method, which should be const

### What changes are included in this PR?

Change `Compare`, `GetMinMax`, `GetMinMaxSpaced` to const

### Are these changes tested?

No

### Are there any user-facing changes?

No

* Closes: #38874

Authored-by: mwish <maplewish117@gmail.com>
Signed-off-by: mwish <maplewish117@gmail.com>