Initial commit of Google Function Adapter

implement HttpFunction
add headers processing
Some refactoring [WIP]
Add invoker integration test
Make test classes nested within test.
Add sample and refdoc

Resolves #468

spring-cloud-function-samples/pom.xml

@@ -24,6 +24,7 @@
 		<module>function-sample-aws-custom</module>
 		<module>function-sample-azure</module>
 		<module>function-sample-spring-integration</module>
+		<module>function-sample-gcp</module>
 	</modules>
 
 	<build>