Allow providing an external validator

Signed-off-by: Ruben Romero Montes <rromerom@redhat.com>

api/src/main/java/io/cloudevents/v03/CloudEventBuilder.java

@@ -42,7 +42,14 @@ import io.cloudevents.fun.EventBuilder;
  */
 public final class CloudEventBuilder<T> implements 
 		EventBuilder<T, AttributesImpl> {
-	private CloudEventBuilder() {}
+
+	private CloudEventBuilder(Validator validator) {
+		if(validator == null) {
+			this.validator = getValidator();
+		} else {
+			this.validator = validator;
+		}
+	}
 
 	private static Validator VALIDATOR;
 	
@@ -65,6 +72,7 @@ public final class CloudEventBuilder<T> implements
 	private T data;
 	
 	private final Set<ExtensionFormat> extensions = new HashSet<>();
+	private final Validator validator;
 	
 	private static Validator getValidator() {
 		if(null== VALIDATOR) {
@@ -78,14 +86,22 @@ public final class CloudEventBuilder<T> implements
 	 * @param <T> The 'data' type
 	 */
 	public static <T> CloudEventBuilder<T> builder() {
-		return new CloudEventBuilder<T>();
+		return new CloudEventBuilder<T>(null);
+	}
+
+	public static <T> CloudEventBuilder<T> builder(Validator validator) {
+		return new CloudEventBuilder<T>(validator);
+	}
+	public static <T> CloudEventBuilder<T> builder(
+			CloudEvent<AttributesImpl, T> base) {
+		return builder(base, null);
 	}
 
 	public static <T> CloudEventBuilder<T> builder(
-			CloudEvent<AttributesImpl, T> base) {
+			CloudEvent<AttributesImpl, T> base, Validator validator) {
 		Objects.requireNonNull(base);
 
-		CloudEventBuilder<T> result = new CloudEventBuilder<>();
+		CloudEventBuilder<T> result = new CloudEventBuilder<>(validator);
 
 		AttributesImpl attributes = base.getAttributes();
 
@@ -194,9 +210,9 @@ public final class CloudEventBuilder<T> implements
 				new CloudEventImpl<T>(attributes, data, extensions);
 		
 		Set<ConstraintViolation<Object>> violations =
-				getValidator().validate(cloudEvent);
+				validator.validate(cloudEvent);
 		
-		violations.addAll(getValidator().validate(cloudEvent.getAttributes()));
+		violations.addAll(validator.validate(cloudEvent.getAttributes()));
 		
 		final String errs = 
 			violations.stream()