apacheGH-39413: [C++][Parquet] Vectorize decode plain on FLBA (apache…

…#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: apache#39413

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

cpp/src/parquet/encoding.cc

@@ -1080,9 +1080,7 @@ inline int DecodePlain<FixedLenByteArray>(const uint8_t* data, int64_t data_size
     ParquetException::EofException();
   }
   for (int i = 0; i < num_values; ++i) {
-    out[i].ptr = data;
-    data += type_length;
-    data_size -= type_length;
+    out[i].ptr = data + i * type_length;
   }
   return static_cast<int>(bytes_to_decode);
 }