psql: Add command to use extended query protocol

psql: Add command to use extended query protocol

This adds a new psql command \bind that sets query parameters and
causes the next query to be sent using the extended query protocol.
Example:

    SELECT $1, $2 \bind 'foo' 'bar' \g

This may be useful for psql scripting, but one of the main purposes is
also to be able to test various aspects of the extended query protocol
from psql and to write tests more easily.

Reviewed-by: Corey Huinker <corey.huinker@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/e8dd1cd5-0e04-3598-0518-a605159fe314@enterprisedb.com