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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/26cb14aea12a0f0c2f9a49de3865721936b711a7

Modified Files
--------------
contrib/file_fdw/data/multiline_header.csv | 4 ++
contrib/file_fdw/expected/file_fdw.out | 29 +++++++++++++-
contrib/file_fdw/sql/file_fdw.sql | 13 +++++++
doc/src/sgml/file-fdw.sgml | 4 +-
src/backend/commands/copy.c | 62 +++++++++++++++++-------------
src/test/regress/expected/copy.out | 18 +++++++++
src/test/regress/expected/copy2.out | 6 +++
src/test/regress/sql/copy.sql | 15 ++++++++
src/test/regress/sql/copy2.sql | 3 ++
9 files changed, 125 insertions(+), 29 deletions(-)