Add docs and updates for background function support

rename fuction-sample-gcp to function-sample-gcp-http

refdoc polish

background sample polish

Resolves #525
Update pub/sub bg function to use base64 encoding

spring-cloud-function-samples/function-sample-gcp-http/src/main/java/com/example/CloudFunctionMain.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2020-2020 the original author or 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
+ *
+ *      https://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 com.example;
+
+import java.util.function.Function;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.context.annotation.Bean;
+
+@SpringBootApplication
+public class CloudFunctionMain {
+
+	public static void main(String[] args) {
+		SpringApplication.run(CloudFunctionMain.class, args);
+	}
+
+	@Bean
+	public Function<String, String> function() {
+		return value -> value.toUpperCase();
+	}
+}