Define a property for the CloudEvents SDK version to avoid mentioning it three times. (GoogleCloudPlatform#80)

* Update the version of the CloudEvents SDK to 2.0.0RC2.

Since the API is hopefully stable now, rename `ExperimentalCloudEventsFunction`
to `CloudEventsFunction`.

Add support for a `FUNCTION_SIGNATURE_TYPE` of `cloudevent`. Treat it the same
as `background`. (We don't really need `FUNCTION_SIGNATURE_TYPE`, since the
interface implemented by the function class tells us what the signature is.)

Simplify the integration test slightly by removing some intermediary helper
methods.

* Define a property for the CloudEvents SDK version to avoid mentioning it three times.

Also fix the import order in a couple of source files.

* Define a property for the CloudEvents SDK version to avoid mentioning it three times.

Also fix the import order in a couple of source files.

Author: eamonnmcmanus

invoker/core/pom.xml

@@ -22,6 +22,7 @@
     <junit.jupiter.version>5.3.2</junit.jupiter.version>
     <maven.compiler.source>11</maven.compiler.source>
     <maven.compiler.target>11</maven.compiler.target>
+    <cloudevents.sdk.version>2.0.0.RC2</cloudevents.sdk.version>
   </properties>
 
   <licenses>
@@ -52,17 +53,17 @@
     <dependency>
       <groupId>io.cloudevents</groupId>
       <artifactId>cloudevents-core</artifactId>
-      <version>2.0.0.RC2</version>
+      <version>${cloudevents.sdk.version}</version>
     </dependency>
     <dependency>
       <groupId>io.cloudevents</groupId>
       <artifactId>cloudevents-http-basic</artifactId>
-      <version>2.0.0.RC2</version>
+      <version>${cloudevents.sdk.version}</version>
     </dependency>
     <dependency>
       <groupId>io.cloudevents</groupId>
       <artifactId>cloudevents-json-jackson</artifactId>
-      <version>2.0.0.RC2</version>
+      <version>${cloudevents.sdk.version}</version>
     </dependency>
     <dependency>
       <groupId>com.google.code.gson</groupId>

invoker/core/src/main/java/com/google/cloud/functions/invoker/BackgroundFunctionExecutor.java

@@ -19,6 +19,7 @@
 import static java.util.stream.Collectors.toMap;
 
 import com.google.cloud.functions.BackgroundFunction;
+import com.google.cloud.functions.CloudEventsFunction;
 import com.google.cloud.functions.Context;
 import com.google.cloud.functions.RawBackgroundFunction;
 import com.google.gson.Gson;
@@ -45,7 +46,6 @@
 import javax.servlet.http.HttpServlet;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
-import com.google.cloud.functions.CloudEventsFunction;
 
 /** Executes the user's background function. */
 public final class BackgroundFunctionExecutor extends HttpServlet {

invoker/core/src/test/java/com/google/cloud/functions/invoker/testfunctions/CloudEventSnoop.java

@@ -2,14 +2,14 @@
 
 import static java.nio.charset.StandardCharsets.UTF_8;
 
+import com.google.cloud.functions.CloudEventsFunction;
 import com.google.gson.Gson;
 import com.google.gson.JsonObject;
 import io.cloudevents.CloudEvent;
 import io.cloudevents.core.format.EventFormat;
 import io.cloudevents.core.provider.EventFormatProvider;
 import io.cloudevents.jackson.JsonFormat;
 import java.io.FileOutputStream;
-import com.google.cloud.functions.CloudEventsFunction;
 
 public class CloudEventSnoop implements CloudEventsFunction {
   @Override