add a transformer to replace column with hardcoded value

[fazo96]

Hi, I originally made this request in #110

What we need is a transformer that can replace all values in a column with a hardcoded value. The main use case we have for it is replacing values in columns that hold financial data such as the total on invoices and prices of individual products.

Most of those columns are of type float, int or bigint on a postgres database, but some are string or text. Ideally, the transformer would work on all those column types.