pgpool: Feature: implement protocol version 3.2 BackendKeyData and quer

Feature: implement protocol version 3.2 BackendKeyData and query cancel message.

Starting from PostgreSQL 18, frontend/backend protocol has been
changed to 3.2. In the changes the BackendKeyData and query cancel
message are modified to allow variable length cancel key.

This commit implements the changes and now we can connect to
PostgreSQL frontend and backend using 3.2 protocol.

Example session is:
PGMAXPROTOCOLVERSION="3.2" psql -p 11000 test

Author: Tatsuo Ishii <ishii(at)postgresql(dot)org>
Discussion: https://www.postgresql.org/message-id/20250714.155710.1706961744888449986.ishii%40postgresql.org

Branch
------
master

Details
-------
https://git.postgresql.org/gitweb?p=pgpool2.git;a=commitdiff;h=523b3d94d9b9bd5e66d547ef2c588e7205522078

Modified Files
--------------
src/auth/pool_auth.c | 85 ++++++++++++++++++-------------
src/context/pool_query_context.c | 8 +--
src/include/pcp/libpcp_ext.h | 8 ++-
src/include/pool.h | 11 ++--
src/include/protocol/pool_process_query.h | 4 +-
src/include/protocol/pool_proto_modules.h | 3 +-
src/protocol/child.c | 70 +++++++++++++++++--------
src/protocol/pool_process_query.c | 39 ++++++++------
src/protocol/pool_proto_modules.c | 2 +-
src/rewrite/pool_lobj.c | 4 +-
10 files changed, 151 insertions(+), 83 deletions(-)