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_

从 issue 中链接的 Parquet 数据页编码文档开始，验证 BYTE_ARRAY 和 FIXED_LEN_BYTE_ARRAY 十进制值的字节序和编码规则。完成标准是记录大十进制数使用 big-endian 还是 little-endian 字节，以及何时会进行编码或字节交换。

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