json format for COPY TO

This introduces the JSON format option for the COPY TO command, allowing
users to export query results or table data directly as a stream of JSON
objects (one per line, NDJSON style).

The JSON format is currently supported only for COPY TO operations; it
is not available for COPY FROM.

JSON format is incompatible with some standard text/CSV formatting
options, including HEADER, DEFAULT, NULL, DELIMITER, FORCE QUOTE,
FORCE NOT NULL, and FORCE NULL.

Column list support is included: when a column list is specified, only
the named columns are emitted in each JSON object.

Regression tests covering valid JSON exports and error handling for
incompatible options have been added to src/test/regress/sql/copy.sql.

Author: Joe Conway <[email protected]>
Author: jian he <[email protected]>
Co-Authored-By: Andrew Dunstan <[email protected]>
Reviewed-by: Andrey M. Borodin <[email protected]>
Reviewed-by: Dean Rasheed <[email protected]>
Reviewed-by: Daniel Verite <[email protected]>
Reviewed-by: Davin Shearer <[email protected]>
Reviewed-by: Masahiko Sawada <[email protected]>
Reviewed-by: Alvaro Herrera <[email protected]>
Reviewed-by: Junwang Zhao <[email protected]>
Discussion: 
https://postgr.es/m/CALvfUkBxTYy5uWPFVwpk_7ii2zgT07t3d-yR_cy4sfrrLU%3Dkcg%40mail.gmail.com
Discussion: 
https://postgr.es/m/[email protected]

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/7dadd38cda95bf5bc0c4715d9ab71766d1693379

Modified Files
--------------
doc/src/sgml/ref/copy.sgml         |  29 ++++++-
src/backend/commands/copy.c        |  53 ++++++++----
src/backend/commands/copyto.c      | 162 ++++++++++++++++++++++++++++++++++---
src/backend/parser/gram.y          |   8 ++
src/backend/utils/adt/json.c       |   5 +-
src/bin/psql/tab-complete.in.c     |   2 +-
src/include/commands/copy.h        |   1 +
src/include/utils/json.h           |   2 +
src/test/regress/expected/copy.out | 146 +++++++++++++++++++++++++++++++++
src/test/regress/sql/copy.sql      |  88 ++++++++++++++++++++
10 files changed, 463 insertions(+), 33 deletions(-)

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