Support multi-line headers in COPY FROM command.

Support multi-line headers in COPY FROM command.

The COPY FROM command now accepts a non-negative integer for the HEADER option,
allowing multiple header lines to be skipped. This is useful when the input
contains multi-line headers that should be ignored during data import.

Author: Shinya Kato <shinya11.kato@gmail.com>
Co-authored-by: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Yugo Nagata <nagata@sraoss.co.jp>
Discussion: http://postgr.es/m/CAOzEurRPxfzbxqeOPF_AGnAUOYf=Wk0we+1LQomPNUNtyZGBZw@mail.gmail.com