Expose unwrap method on SmartValidator

See spring-projectsgh-31082

spring-context/src/main/java/org/springframework/validation/SmartValidator.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -64,4 +64,19 @@ default void validateValue(
 		throw new IllegalArgumentException("Cannot validate individual value for " + targetType);
 	}
 
+	/**
+	 * Return a contained validator instance of the specified type, unwrapping
+	 * as far as necessary.
+	 * @param type the class of the object to return
+	 * @param <T> the type of the object to return
+	 * @return a validator instance of the specified type; {@code null} if there
+	 * isn't a nested validator; an exception may be raised if the specified
+	 * validator type does not match.
+	 * @since 6.1
+	 */
+	@Nullable
+	default <T> T unwrap(@Nullable Class<T> type) {
+		return null;
+	}
+
 }