[PATCH] Add pg_get_subscription_ddl() function

Hi Hackers,

I am submitting a patch as a part of a larger Retail DDL functions
project described by Andrew Dunstan here
<https://www.postgresql.org/message-id/945db7c5-be75-45bf-b55b-cb1e56f2e3e9%40dunslane.net>
. This patch creates a
function pg_get_subscription_ddl, designed to retrieve the full DDL
statement for a subscription. Users can obtain the DDL by providing
the subscription name, like so:

SELECT pg_get_subscription_ddl('testsub1');

pg_get_subscription_ddl

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CREATE SUBSCRIPTION testsub1 CONNECTION 'dbname=db_doesnotexist'
PUBLICATION "testpub1" WITH (connect = false, slot_name = 'testsub1',
enabled = false, binary = false, streaming = parallel, synchronous_commit =
off, two_phase = off, disable_on_error = off, password_required = on,
run_as_owner = off, origin = any, failover = off, retain_dead_tuples = off,
max_retention_duration = 0);
(1 row)

This patch includes documentation, comments, and regression tests.

Regards,
Vaibhav Dalvi
EnterpriseDB