Feature: implement NegotiateProtocolVersion message.

Implementing the message is necessary when frontend requests the
protocol version 3.2 (i.e. PostgreSQL 18+ or compatible clients),
while backend still only supports 3.0 (i.e. backend is PostgreSQL 17
or before).

This commit handles the message so that the message is forwarded from
backend to frontend when there's no connection cache exists.

If connection cache exists, pgpool sends the message, which has been
saved at the time when the connection cache was created, to frontend.

Note that the frontend/backend protocol 3.2 changes the BackendKeyData
message format, but it's not implemented in this commit yet. This
means that still pgpool cannot handle 3.2 protocol.

Discussion: https://www.postgresql.org/message-id/20250708.112133.1324153277751075866.ishii%40postgresql.org

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