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GH-39525: [C++][Parquet] Pass memory pool to decoders (#39526)

### Rationale for this change

Memory pools should be plumbed through where ever possible.

### What changes are included in this PR?

Pass through memory pool to decoders

### Are these changes tested?

Not directly; this was caught via some internal fuzz targets.

### Are there any user-facing changes?

No.

* Closes: #39525

Authored-by: Micah Kornfield <micahk@google.com>
Signed-off-by: mwish <maplewish117@gmail.com>