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-background/src/main/java/com/example/BackgroundFunctionMain.java

@@ -0,0 +1,31 @@
+package com.example;
+
+import java.nio.charset.StandardCharsets;
+import java.util.Base64;
+import java.util.function.Consumer;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.context.annotation.Bean;
+
+@SpringBootApplication
+public class BackgroundFunctionMain {
+
+	public static void main(String[] args) {
+		SpringApplication.run(BackgroundFunctionMain.class, args);
+	}
+
+	/**
+	 * The background function which triggers on an event from Pub/Sub and consumes the Pub/Sub
+	 * event message.
+	 */
+	@Bean
+	public Consumer<PubSubMessage> pubSubFunction() {
+		return message -> {
+			// The PubSubMessage data field arrives as a base-64 encoded string and must be decoded.
+			// See: https://cloud.google.com/functions/docs/calling/pubsub#event_structure
+			String decodedMessage = new String(Base64.getDecoder().decode(message.getData()), StandardCharsets.UTF_8);
+			System.out.println("Received Pub/Sub message with data: " + decodedMessage);
+		};
+	}
+}

spring-cloud-function-samples/function-sample-gcp-background/src/main/java/com/example/PubSubMessage.java

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+package com.example;
+
+import java.util.Map;
+
+/**
+ * A class that can be mapped to the GCF Pub/Sub Message event type. This is for use in
+ * the background functions.
+ *
+ * <p>See the PubSubMessage definition for reference:
+ * https://cloud.google.com/pubsub/docs/reference/rest/v1/PubsubMessage
+ *
+ * @author Mike Eltsufin
+ */
+public class PubSubMessage {
+
+	private String data;
+
+	private Map<String, String> attributes;
+
+	private String messageId;
+
+	private String publishTime;
+
+	public String getData() {
+		return data;
+	}
+
+	public void setData(String data) {
+		this.data = data;
+	}
+
+	public Map<String, String> getAttributes() {
+		return attributes;
+	}
+
+	public void setAttributes(Map<String, String> attributes) {
+		this.attributes = attributes;
+	}
+
+	public String getMessageId() {
+		return messageId;
+	}
+
+	public void setMessageId(String messageId) {
+		this.messageId = messageId;
+	}
+
+	public String getPublishTime() {
+		return publishTime;
+	}
+
+	public void setPublishTime(String publishTime) {
+		this.publishTime = publishTime;
+	}
+
+}

spring-cloud-function-samples/function-sample-gcp-background/src/main/resources/META-INF/MANIFEST.MF

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+Main-Class: com.example.BackgroundFunctionMain