Add Decimal128 API and use it in DecimalArray and DecimalBuilder

[viirya]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Part of #131.

Currently we directly use i128 in decimal related APIs, for example DecimalArray and DecimalBuilder APIs. However, in order to support 256-bit decimal, we need more consistent Decimal API. For example, in C++ Arrow, there are Decimal128 and Decimal256 classes.

This proposes to have Decimal128 API and use it in DecimalArray and DecimalBuilder.

Describe the solution you'd like

Add Decimal128 API and use it in DecimalArray and DecimalBuilder.

Describe alternatives you've considered
Keep i128 in DecimalArray and DecimalBuilder. But there will be inconsistency between Decimal256 related APIs and also inconsistency between C++ Arrow and Rust Arrow APIs.

Additional context
Add any other context or screenshots about the feature request here.