Move DecimalArray to a new file

[HaoYang670]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
The DecimalArray is currently defined in array_binary.rs, which doesn't make sense to me.
Based on the definition : https://github.com/apache/arrow/blob/master/format/Schema.fbs#L181-L195
Decimal Array is a logical type whose physical layout is more like a primitive array.

This is the reason of moving Decimal Array out array_binary.rs.

Describe the solution you'd like
Create a new file array_decimal.rs and move the decimal array here.

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

Additional context
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