Skip text->binary conversion of unnecessary columns in contrib/file_fdw.

When reading from a text- or CSV-format file in file_fdw, the datatype
input routines can consume a significant fraction of the runtime.
Often, the query does not need all the columns, so we can get a useful
speed boost by skipping I/O conversion for unnecessary columns.

To support this, add a "convert_selectively" option to the core COPY code.
This is undocumented and not accessible from SQL (for now, anyway).

Etsuro Fujita, reviewed by KaiGai Kohei