Support type conversion shims in RowSetWriter

The RowSet tools include a RowSetWriter for populating a batch of vectors. A set of "column writers" exist: one for each kind of vector. These classes provide methods to write a value into a vector. For example, the VarcharColumnWriter provides a setString()) method to set the value.

The current writers provide only "natural" conversions: from Java String to Varchar, from Java Double to FLOAT8 and so on. That is, the methods implemented for each type are those that provide s single, unambiguous conversion.

This ticket asks to add a translation layer: to allow, say, writing an Int column using a String (parsed according to some rules). Or, to convert from a string to a Date using some format.

The goal is not to provide the type conversions themselves, rather it is to provide a way to insert the type conversion "shim" on top of the "native" column writer in a way that is transparent to code using the row set writer.