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GH-45126: [C++][Parquet] Fix undefined behavior in the FormatStatValue (#45127)

### Rationale for this change

FormatStatValue function in the parquet/types.cc employs reinterpret_cast to cast bytes to specific data types. It is undefined behavior when the bytes are unaligned.

### What changes are included in this PR?

Use std::memcpy to replace reinterpret_cast.

### Are these changes tested?

Pass CI.

### Are there any user-facing changes?

No.
* GitHub Issue: #45126

Authored-by: Gang Wu <ustcwg@gmail.com>
Signed-off-by: Gang Wu <ustcwg@gmail.com>