MINOR: Fix Embedded ConfigDef Validator toString issue (#6339)

`ConfigDef.embeddedValidator` should return an Anonymous Object instead of lambda so that we can have a useful `toString()` for methods such as `toRst`.

Reviewers: Jason Gustafson <jason@confluent.io>

clients/src/main/java/org/apache/kafka/common/config/ConfigDef.java

@@ -1318,7 +1318,16 @@ public class ConfigDef {
      */
     private static Validator embeddedValidator(final String keyPrefix, final Validator base) {
         if (base == null) return null;
-        return (name, value) -> base.ensureValid(name.substring(keyPrefix.length()), value);
+        return new Validator() {
+            public void ensureValid(String name, Object value) {
+                base.ensureValid(name.substring(keyPrefix.length()), value);
+            }
+
+            @Override
+            public String toString() {
+                return base.toString();
+            }
+        };
     }
 
     /**