Is big Decimal in Parquet big endian?

### Describe the usage question you have. Please include as many useful details as possible.

It's not clear from description https://parquet.apache.org/docs/file-format/data-pages/encodings/ how are big Decimals (precision >= 18) placed. As I could understand they can be encoded as BYTE_ARRAY or FIXED_LEN_BYTE_ARRAY. But there're two ways to place "the bytes contained in the array" from memory: big endian and little endian. I.e. they are in LE in int128 in memory. Could I place the bytes into FIXED_LEN_BYTE_ARRAY as is?

It's also not clean if there's some encoding over these decimal's bytes, should the data be swapped to correct endianness before of after the encoding?

### Component(s)

_No response_

從 issue 中連結的 Parquet 資料頁編碼文件開始，驗證 BYTE_ARRAY 和 FIXED_LEN_BYTE_ARRAY 十進位值的位元組順序與編碼規則。完成標準是記錄大型十進位數使用 big-endian 還是 little-endian 位元組，以及何時會進行編碼或位元組交換。

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