Rename DecimalArray/DecimalBuilder to Decimal128Array/Decimal128Builder

[viirya]

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To be more clear on Decimal APIs and align with other Arrow implementations like C++, Python, we should rename DecimalArray/DecimalBuilder to Decimal128Array/Decimal128Builder.

Python: https://arrow.apache.org/docs/python/generated/pyarrow.Decimal128Array.html?highlight=decimal128array#pyarrow.Decimal128Array
C++: https://arrow.apache.org/docs/cpp/api/array.html?highlight=decimal128array#_CPPv4N5arrow15Decimal128ArrayE

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