SQL/JSON json_table plan clause

Hi hackers!

This thread is further work continued in [1], where Amit Langote
suggested starting discussion on the remaining SQL/JSON feature
'PLAN clause for JSON_TABLE' anew.

We'd like to help with merging SQL/JSON patches into vanilla,
and have adapted PLAN clause to recent changes in JSON_TABLE
function.

While doing this we've found that some tests with the PLAN clause
were incorrect, along with JSON_TABLE behavior with this clause.
We've corrected this behavior, but these corrections required reverting
some removed and heavily refactored code, so we'd be glad for review
and feedback on this patch.

Also, we've noticed that Oracle set the PLAN clause for JSON_TABLE
aside for some reason.

[1]
https://www.postgresql.org/message-id/flat/CA%2BHiwqE4XTdfb1nW%3DOjoy_tQSRhYt-q_kb6i5d4xcKyrLC1Nbg%40mail.gmail.com

-- 
Regards,
Nikita Malakhov
Postgres Professional
The Russian Postgres Company
https://postgrespro.ru/