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 133.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

Test.

Branch
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V4_7_STABLE

Details
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https://git.postgresql.org/gitweb?p=pgpool2.git;a=commitdiff;h=08fbe906db7f751dca4a9aa9c44e688e653f32dc

Modified Files
--------------
src/auth/pool_auth.c | 86 ++++++++++++++++++-----
src/test/regression/tests/133.md5_warning/test.sh | 70 ++++++++++++++++++
2 files changed, 137 insertions(+), 19 deletions(-)