Implement SqlTypeDecimalDigits in type_traits

DECIMAL_DIGITS: The total number of significant digits to the right of the decimal point. 

for details, see https://docs.microsoft.com/en-us/sql/odbc/reference/syntax/sqlcolumns-function?view=sql-server-ver15

Start by locating the type_traits implementation and reading how existing SQL metadata traits are defined. Consult the linked SQLColumns documentation for DECIMAL_DIGITS, then confirm that the new SqlTypeDecimalDigits trait reports the specified number of digits to the right of the decimal point.

Written by the indexing model from the issue text.