file_fdw: Support multi-line HEADER option.

file_fdw: Support multi-line HEADER option.

Commit bc2f348 introduced multi-line HEADER support for COPY. This commit
extends this capability to file_fdw, allowing multiple header lines to be
skipped.

Because CREATE/ALTER FOREIGN TABLE requires option values to be single-quoted,
this commit also updates defGetCopyHeaderOption() to accept integer values
specified as strings for HEADER option.

Author: Shinya Kato <shinya11.kato@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: songjinzhou <tsinghualucky912@foxmail.com>
Reviewed-by: Japin Li <japinli@hotmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAOzEurT+iwC47VHPMS+uJ4WSzvOLPsZ2F2_wopm8M7O+CZa3Xw@mail.gmail.com