pgsql: Make pg_createsubscriber warn if publisher has two-phase commit

Make pg_createsubscriber warn if publisher has two-phase commit enabled.

pg_createsubscriber currently always sets up logical replication
with two-phase commit disabled. Improving that is not going to
happen for v17. In the meantime, document the deficiency, and
adjust pg_createsubscriber so that it will emit a warning if
the source installation has max_prepared_transactions > 0.

Hayato Kuroda (some mods by Amit Kapila and me), per complaint from
Noah Misch

Discussion: https://postgr.es/m/20240623062157.97.nmisch@google.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/917754557cc0002bb042341720a7ce18fe5b0a09

Modified Files
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doc/src/sgml/ref/pg_createsubscriber.sgml | 11 ++++++++++
src/bin/pg_basebackup/pg_createsubscriber.c | 34 ++++++++++++++---------------
2 files changed, 28 insertions(+), 17 deletions(-)