support for PREPARE and EXECUTE statement

[TwistingTwists]

Sister PR: sunng87/pgwire#409

Closes: #292

-- Run inside psql connected to datafusion-postgres:
--   psql -h 127.0.0.1 -p 15432

-- 1. Basic PREPARE/EXECUTE
PREPARE my_stmt AS SELECT 42 AS answer;
EXECUTE my_stmt;
EXECUTE my_stmt;

-- 2. Parameterized
PREPARE param_stmt (INT) AS SELECT $1 * 2 AS doubled;
EXECUTE param_stmt(21);
EXECUTE param_stmt(100);

-- 3. DEALLOCATE specific (my_stmt gone, param_stmt still works)
DEALLOCATE my_stmt;
EXECUTE param_stmt(5);

-- 4. DEALLOCATE ALL
PREPARE stmt_a AS SELECT 1;
PREPARE stmt_b AS SELECT 2;
DEALLOCATE ALL;

-- 5. These should all error with "does not exist"
EXECUTE stmt_a;
EXECUTE stmt_b;
EXECUTE param_stmt(1);
EXECUTE never_prepared;

Prepared Statement Test Results

All tests passed end-to-end with real psql.

Test	Result
PREPARE my_stmt AS SELECT 42	✅ PREPARE
EXECUTE my_stmt (twice)	✅ Returns 42 both times
PREPARE param_stmt (INT) AS SELECT $1 * 2	✅ PREPARE
EXECUTE param_stmt(21)	✅ Returns 42
EXECUTE param_stmt(100)	✅ Returns 200
DEALLOCATE my_stmt	✅ DEALLOCATE
EXECUTE my_stmt after deallocate	✅ Error: "does not exist"
DEALLOCATE ALL	✅ DEALLOCATE
EXECUTE stmt_a after DEALLOCATE ALL	✅ Error: "does not exist"