pgsql: Raise a WARNING for missing publications.

Raise a WARNING for missing publications.

When we create or alter a subscription to add publications raise a warning
for non-existent publications. We don't want to give an error here because
it is possible that users can later create the missing publications.

Author: Vignesh C
Reviewed-by: Bharath Rupireddy, Japin Li, Dilip Kumar, Euler Taveira, Ashutosh Sharma, Amit Kapila
Discussion: https://postgr.es/m/CALDaNm0f4YujGW+q-Di0CbZpnQKFFrXntikaQQKuEmGG0=Zw=Q@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8f2e2bbf145384784bad07a96d461c6bbd91f597

Modified Files
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doc/src/sgml/ref/alter_subscription.sgml  |   4 +-
doc/src/sgml/ref/create_subscription.sgml |   7 ++
src/backend/commands/subscriptioncmds.c   | 133 +++++++++++++++++++++++++-----
src/test/subscription/t/007_ddl.pl        |  37 +++++++++
4 files changed, 160 insertions(+), 21 deletions(-)