Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Why I prefer using const_generic on Decimal?

• Decimal128 and Decimal256 are different datatype. However all of their fields have same data type except the length of the value buffer [u8; 16] or [u8; 32]. This is the situation which const_generic is very suitable.
• In the Rust standard library, there is a famous type using const_generic: the Array type: https://doc.rust-lang.org/std/primitive.array.html Decimal is very similar to Array
• Remove some repeated code.
• More constant evaluation
• Easy to extend: We might add more Decimal type in the future. For example in arrow2, there is a PR to add Decimal 32 and 64: [Requires Arrow spec] Added support for decimal 32 and 64 jorgecarleitao/arrow2#896

Will the rewriting leads to many API changes?
Yes, But for users, the answer is no. Because this is just an internal refactor.

Checked List

• BasicDecimal (Rewrite Decimal and DecimalArray using const_generic #2383 )
• BasicDecimalArray (Rewrite Decimal and DecimalArray using const_generic #2383 )
• Rewriting Decimal Builders using const_generic. #2390

Describe alternatives you've considered
We could not do this.

Additional context
Related to #2362 and #2274.