Add COPY (on_error set_null) option

Add COPY (on_error set_null) option

If ON_ERROR SET_NULL is specified during COPY FROM, any data type
conversion errors will result in the affected column being set to a
null value.  A column's not-null constraints are still enforced, and
attempting to set a null value in such columns will raise a constraint
violation error.  This applies to a column whose data type is a domain
with a NOT NULL constraint.

Author: Jian He <jian.universality@gmail.com>
Author: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@oss.nttdata.com>
Reviewed-by: Jim Jones <jim.jones@uni-muenster.de>
Reviewed-by: "David G. Johnston" <david.g.johnston@gmail.com>
Reviewed-by: Yugo NAGATA <nagata@sraoss.co.jp>
Reviewed-by: torikoshia <torikoshia@oss.nttdata.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Atsushi Torikoshi <torikoshia@oss.nttdata.com>
Reviewed-by: Matheus Alcantara <matheusssilv97@gmail.com>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://www.postgresql.org/message-id/flat/CAKFQuwawy1e6YR4S%3Dj%2By7pXqg_Dw1WBVrgvf%3DBP3d1_aSfe_%2BQ%40mail.gmail.com