feat: Divert cast of PostgreSQL decimal with scale>0 to to_char

[nj1973]

Example of PostgreSQL default CAST behaviour (note the unnecessary .00):

# SELECT CAST(CAST(100 AS DECIMAL(5,2)) AS VARCHAR(10));
 varchar
---------
 100.00

Most other engines would return a string of '100' from the above operation.

This change overrides the PostgreSQL cast() function in third_party/ibis/ibis_addon/operations.py. The new code diverts the operation to rtrim(to_char(),'.') for Decimal columns with a scale > 0. The example about then looks like this:

# SELECT RTRIM(TO_CHAR(CAST(100 AS DECIMAL(5,2)),'FM999.99'),'.');
 rtrim
-------
 100

[nj1973]

/gcbrun

[nehanene15]

LGTM! This is great.