Added Decimal256 Type Support in Dictionaries (Experimental Feature)

Why it matters

This feature enables the use of the Decimal256 type within dictionaries to handle very high precision decimal numbers. It addresses the need for storing and querying large decimal values accurately, expanding ClickHouse's capabilities for financial, scientific, and other precision-critical applications.

How to use it

To use this feature, define dictionary attributes or keys with the Decimal256 data type in your dictionary configuration. Note that Decimal256 support is currently experimental, so it is recommended to test carefully before using it in production.