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GH-36643: [C++][Parquet] Use nested namespace in parquet (#36647)



### Rationale for this change

Just for code style, using nested namespace rather than before

### What changes are included in this PR?

Using nested namespace

### Are these changes tested?

no

### Are there any user-facing changes?

no

* Closes: #36643

Authored-by: mwish <maplewish117@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>