Add public factory method to SubselectExpression.

This allows using Select instances as Expression where this isn't yet anticipated.

Closes #1831

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/core/mapping/schema/LiquibaseChangeSetWriter.java

@@ -92,6 +92,7 @@ import org.springframework.util.Assert;
  * @author Kurt Niemi
  * @author Mark Paluch
  * @author Evgenii Koba
+ * @author Jens Schauder
  * @since 3.2
  */
 public class LiquibaseChangeSetWriter {