fix: make IntervalQualifier serializable (spotbugs SE_BAD_FIELD)

Signed-off-by: 付典 <fudianchn@gmail.com>

Author: fudianchn

src/main/java/net/sf/jsqlparser/expression/IntervalExpression.java

@@ -11,6 +11,7 @@
 
 import net.sf.jsqlparser.parser.ASTNodeAccessImpl;
 
+import java.io.Serializable;
 import java.util.Objects;
 
 public class IntervalExpression extends ASTNodeAccessImpl implements Expression {
@@ -121,7 +122,7 @@ public <E extends Expression> E getExpression(Class<E> type) {
      * {@code INTERVAL} column / cast target type, used in DDL), so the qualifier is represented
      * once and round-trips consistently across all contexts.
      */
-    public static class IntervalQualifier {
+    public static class IntervalQualifier implements Serializable {
 
         private final String leadingField;
         private final Integer leadingFieldPrecision;