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

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@postgresql.org>
Discussion: https://www.postgresql.org/message-id/20250714.155710.1706961744888449986.ishii%40postgresql.org