pgpool: Fix pool_do_auth.

Fix pool_do_auth.

From PostgreSQL 19, backend sends a notice message "authenticated with
an MD5-encrypted password" if MD5 auth is used. Unfortunately it
exercises an untested code path in pool_do_auth. It calls
SimpleForwardToFrontend to forward the notice message to
frontend. Unfortunately calling it in authentication phase is wrong,
since "session contest" is not established yet. If the session context
is not established, SimpleForwardToFrontend fails with FATAL.

To fix this, create a new function "forward_message_to_frontend",
which is basically a simple version of SimpleForwardToFrontend error,
so that it does not raise the FATAL error.

Also add 083.md5_warning test for this.

Reported-by: harudini
Author: Tatsuo Ishii <ishii(at)postgresql(dot)org>
Discussion: https://github.com/pgpool/pgpool2/issues/168
Backpatch-through: v4.3

add test

Branch
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V4_3_STABLE

Details
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https://git.postgresql.org/gitweb?p=pgpool2.git;a=commitdiff;h=27b933fe9dedfbe57f8d68f2043e9fbd9b4ef0dc

Modified Files
--------------
src/auth/pool_auth.c | 86 ++++++++++++++++++-----
src/test/regression/tests/083.md5_warning/test.sh | 70 ++++++++++++++++++
2 files changed, 137 insertions(+), 19 deletions(-)