added constraints and constraint annotations, except foreign key annotation

src/main/java/jef/model/constraints/KeyConstraint.java

@@ -0,0 +1,21 @@
+package jef.model.constraints;
+
+import jef.model.DbEntity;
+import jef.model.DbField;
+import lombok.AllArgsConstructor;
+import lombok.Getter;
+
+import java.util.List;
+import java.util.stream.Collectors;
+
+@Getter
+@AllArgsConstructor
+public class KeyConstraint implements Constraint {
+    private final DbEntity<?> entity;
+    private final List<DbField<?>> fields;
+
+    @Override
+    public String getName() {
+        return "K_" + entity.getName() + "_" + fields.stream().map(DbField::getName).collect(Collectors.joining("_"));
+    }
+}