Support decimal data type in arrow

[nickolay]

Describe your feature request

Columns of type DECIMAL in the database should be kept DECIMAL in arrow/polars instead of being converted to FLOAT/DOUBLE.

Motivation: when porting existing code from MSSQL to polars/duckdb, I expected the rewrite to produce the exact same results, but float/decimal differences prevented this from happening. I had to cast back to decimal in duckdb manually.

Currently

• DECIMAL is defined in multiple source typesystems (for example, mssql)
• but not in destinations/arrow, despite decimal being supported by the arrow spec and polars-arrow
• so Decimals are converted to Float64 in the transport