Envelope unmarshaller funcional interface

Signed-off-by: Fabio José <fabiojose@gmail.com>

api/src/main/java/io/cloudevents/fun/EnvelopeUnmarshaller.java

@@ -0,0 +1,35 @@
+/**
+ * Copyright 2019 The CloudEvents Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.cloudevents.fun;
+
+import io.cloudevents.Attributes;
+import io.cloudevents.CloudEvent;
+
+/**
+ * 
+ * @author fabiojose
+ *
+ */
+public interface EnvelopeUnmarshaller<A extends Attributes, T, P> {
+
+	/**
+	 * Unmarshals the payload into {@link CloudEvent} instance impl 
+	 * @param payload The envelope payload
+	 * @return The unmarshalled impl of CloudEvent
+	 */
+	CloudEvent<A, T> unmarshal(P payload);
+	
+}