Introducing INT256 Physical Type in Velox for Enhanced Decimal Precision

[xiaodouchen]

I am considering the addition of a new physical type INT256 to Velox to support higher precision for decimals, analogous to ClickHouse's decimal256 (https://github.com/ClickHouse/ClickHouse/blob/f2bad692e2835c58807daba0d4fa21eb4f440c1d/base/base/Decimal.h#L21).

I see two potential approaches to introduce int256:

1. Directly adding a new physical type INT256 to Physical Type.
2. Indirectly supporting it through the OPAQUE type. However, I've noticed that the OPAQUE type has limited support across various components, such as OPAQUE type is not supported in RowContainer. This limitation presents a challenge for the integration.

Do you have any recommendations on the best approach to implement int256? Or perhaps there's an alternative solution that I haven't considered?

Additionally, I would like to inquire about the specific modules that require updates to accommodate a new physical type and its associated logical types. So far, I am aware of the need to update functions, serialization, RowContainer, and vectors. Are there any other modules I might be overlooking? Could you provide any guidance or suggestions?

cc: @mbasmanova @majetideepak @karteekmurthys