Add GetString to ColumnDecimal.

[theory]

It converts the internal integer to a decimal string relative to the scale and precision. Not a blocker for my projects (I copied the code), but I am getting an error when using -D BUILD_SHARED_LIBS:

undefined symbol: _ZN4absllsERSoNS_6int128E

Presumably -D BUILD_SHARED_LIBS needs to also build and install the absl library.

[Copilot]

Pull request overview

This PR adds a GetString method to the ColumnDecimal class that converts internal integer representations to human-readable decimal strings with proper scale and precision formatting.

Key Changes:

• Added GetString method to ColumnDecimal class that formats decimal values as strings
• Added comprehensive test coverage for the new method across various decimal types and edge cases

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
clickhouse/columns/decimal.h	Declares the new GetString method with documentation
clickhouse/columns/decimal.cpp	Implements decimal-to-string conversion logic with proper handling of scale, precision, and negative values
ut/client_ut.cpp	Adds extensive test cases validating the string conversion for positive, negative, and edge-case decimal values

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