pgsql: Refactor code converting a publication name List to a StringInfo

Refactor code converting a publication name List to a StringInfo

The existing get_publications_str() is renamed to GetPublicationsStr()
and is moved to pg_subscription.c, so as it is possible to reuse it at
two locations of the tablesync code where the same logic was duplicated.

fetch_remote_table_info() was doing two List->StringInfo conversions
when dealing with a server of version 15 or newer. The conversion
happens only once now.

This refactoring leads to less code overall.

Author: Peter Smith
Reviewed-by: Michael Paquier, Masahiko Sawada
Discussion: https://postgr.es/m/CAHut+PtJMk4bKXqtpvqVy9ckknCgK9P6=FeG8zHF=6+Em_Snpw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/248c2d19238c7616531caa8b3101cca4f656185c

Modified Files
--------------
src/backend/catalog/pg_subscription.c | 31 +++++++++++++++
src/backend/commands/subscriptioncmds.c | 60 +++++++----------------------
src/backend/replication/logical/tablesync.c | 35 +++++------------
src/include/catalog/pg_subscription.h | 5 ++-
4 files changed, 58 insertions(+), 73 deletions(-)