Add a function type for functions that receive CloudEvents. (#51)

This is not yet connected to anything.

functions-framework-api/pom.xml

@@ -48,6 +48,14 @@
     <tag>HEAD</tag>
   </scm>
 
+  <dependencies>
+    <dependency>
+      <groupId>io.cloudevents</groupId>
+      <artifactId>cloudevents-api</artifactId>
+      <version>2.0.0-milestone1</version>
+    </dependency>
+  </dependencies>
+
   <build>
     <plugins>
       <plugin>

functions-framework-api/src/main/java/com/google/cloud/functions/ExperimentalCloudEventsFunction.java

@@ -0,0 +1,22 @@
+package com.google.cloud.functions;
+
+import io.cloudevents.CloudEvent;
+
+/**
+ * Represents a Cloud Function that is activated by an event and parsed into a {@link CloudEvent} object.
+ * Because the {@link CloudEvent} API is not yet stable, a function implemented using this class may not
+ * build or work correctly with later versions of that API. Once the API is stable, this interface will
+ * become {@code CloudEventsFunction} and will also be stable.
+ */
+@FunctionalInterface
+public interface ExperimentalCloudEventsFunction {
+  /**
+   * Called to service an incoming event. This interface is implemented by user code to
+   * provide the action for a given background function. If this method throws any exception
+   * (including any {@link Error}) then the HTTP response will have a 500 status code.
+   *
+   * @param event the event.
+   * @throws Exception to produce a 500 status code in the HTTP response.
+   */
+  void accept(CloudEvent event) throws Exception;
+}