Support custom conversion of result data

[jraymakers]

Result data can be converted to JSON-serializable JS objects using any of the variants of get...Json (e.g. getRowObjectsJson).

However, it would be useful to be able to specify custom conversions. This is already somewhat supported at the chunk level through the various convert... methods and the DuckDBValueConverter interface (and indeed this is what the get...Json methods use). But it would be more convenient to have top-level methods on DuckDBResult and DuckDBResultReader to get data using a custom converter.

Additionally, it would be helpful to be able to specify custom conversion by starting from an existing, library-supplied converter and only specifying the (usually few) differences desired.

One example of doing this is the pg-promise API:

function setPgParserTypes() {
  // see https://stackoverflow.com/questions/39168501/pg-promise-returns-integers-as-strings
  pgp().pg.types.setTypeParser(TypeId.INT8, parseInt);
  pgp().pg.types.setTypeParser(TypeId.FLOAT8, parseFloat);
  pgp().pg.types.setTypeParser(TypeId.NUMERIC, parseFloat);
}

(From https://stackoverflow.com/questions/39168501/pg-promise-returns-integers-as-strings)