pgsql: Add option force_array for COPY JSON FORMAT

Add option force_array for COPY JSON FORMAT

This adds the force_array option, which is available exclusively
when using COPY TO with the JSON format.

When enabled, this option wraps the output in a top-level JSON array
(enclosed in square brackets with comma-separated elements), making the
entire result a valid single JSON value.  Without this option, the
default behavior is to output a stream of independent JSON objects.

Attempting to use this option with COPY FROM or with formats other than
JSON will raise an error.

Author: Joe Conway <mail@joeconway.com>
Author: jian he <jian.universality@gmail.com>
Reviewed-by: Junwang Zhao <zhjwpku@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Florents Tselai <florents.tselai@gmail.com>
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://postgr.es/m/CALvfUkBxTYy5uWPFVwpk_7ii2zgT07t3d-yR_cy4sfrrLU%3Dkcg%40mail.gmail.com
Discussion: https://postgr.es/m/6a04628d-0d53-41d9-9e35-5a8dc302c34c@joeconway.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/4c0390ac53b745c2800f6aa3d9ee2515d6ab499b

Modified Files
--------------
doc/src/sgml/ref/copy.sgml         | 30 ++++++++++++++++++++++++++++++
src/backend/commands/copy.c        | 13 +++++++++++++
src/backend/commands/copyto.c      | 38 ++++++++++++++++++++++++++++++++++++--
src/bin/psql/tab-complete.in.c     |  2 +-
src/include/commands/copy.h        |  1 +
src/test/regress/expected/copy.out | 37 +++++++++++++++++++++++++++++++++++++
src/test/regress/sql/copy.sql      | 13 +++++++++++++
7 files changed, 131 insertions(+), 3 deletions(-)