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GH-39413: [C++][Parquet] Vectorize decode plain on FLBA (#39414)



### Rationale for this change

### What changes are included in this PR?
FLBA Decode Plain is not vectorized. So this parsing can be implemented faster https://godbolt.org/z/xWeb93xjW

### Are these changes tested?
Yes, on unittest

### Are there any user-facing changes?

* Closes: #39413

Authored-by: Dmitry Stasenko <dmitry.stasenko@pinely.com>
Signed-off-by: mwish <maplewish117@gmail.com>