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