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

src/test/regress/sql/psql.sql

@@ -45,6 +45,20 @@ SELECT 1 as one, 2 as two \g (format=csv csv_fieldsep='\t')
 SELECT 1 as one, 2 as two \gx (title='foo bar')
 \g
 
+-- \bind (extended query protocol)
+
+SELECT 1 \bind \g
+SELECT $1 \bind 'foo' \g
+SELECT $1, $2 \bind 'foo' 'bar' \g
+
+-- errors
+-- parse error
+SELECT foo \bind \g
+-- tcop error
+SELECT 1 \; SELECT 2 \bind \g
+-- bind error
+SELECT $1, $2 \bind 'foo' \g
+
 -- \gset
 
 select 10 as test01, 20 as test02, 'Hello' as test03 \gset pref01_