Add ALTER TABLE ADD CONSTRANT DDL to support defining primary key and foreign keys

ALTER TABLE [db_name.]table_name ADD [CONSTRAINT constraintName]
  (PRIMARY KEY (col_names) |
  FOREIGN KEY (col_names) REFERENCES [db_name.]table_name [(col_names)])
  [VALIDATE | NOVALIDATE] [RELY | NORELY]

ALTER TABLE employee _ADD CONSTRANT pk_ PRIMARY KEY(empno) VALIDATE RELY
ALTER TABLE department _ADD CONSTRAINT emp_fk_ FOREIGN KEY (mgrno) REFERENCES employee(empno) NOVALIDATE NORELY

ALTER TABLE department ADD PRIMARY KEY(deptno) VALIDATE RELY
ALTER TABLE employee ADD FOREIGN KEY (workdept) REFERENCES department(deptno) VALIDATE RELY;