GH-750 Add support for pluggable protobufs

This initial support adds plugin extension to support CloudEvent proto as well as the example
Additional plugins could be provided in the same ay as CloudEvent plugin extension

Resolves #750

spring-cloud-function-samples/function-sample-grpc-cloudevent/src/main/java/com/example/grpc/demo/DemoGrpcApplication.java

@@ -0,0 +1,65 @@
+package com.example.grpc.demo;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Function;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.cloud.function.grpc.MessagingServiceGrpc;
+import org.springframework.context.ApplicationContext;
+import org.springframework.context.annotation.Bean;
+import org.springframework.messaging.Message;
+import org.springframework.messaging.support.MessageBuilder;
+
+import com.google.protobuf.DescriptorProtos.FileDescriptorProto;
+import com.google.protobuf.DescriptorProtos.FileDescriptorSet;
+import com.google.protobuf.Descriptors.FileDescriptor;
+import com.google.protobuf.ProtocolStringList;
+
+import io.cloudevents.v1.CloudEventServiceGrpc;
+import io.cloudevents.v1.proto.CloudEvent;
+import io.cloudevents.v1.proto.CloudEvent.CloudEventAttributeValue;
+import io.grpc.ManagedChannel;
+import io.grpc.ManagedChannelBuilder;
+
+@SpringBootApplication
+public class DemoGrpcApplication  {
+
+	public static void main(String[] args) throws Exception {
+
+		SpringApplication.run(DemoGrpcApplication.class, args);
+
+		CloudEvent cloudEvent = CloudEvent.newBuilder()
+				.setTextData("{\"event_name\":\"SCF supports CloudEvent gRPC\"}")
+				.setSource("http://springsource.com")
+				.setId("12345")
+				.setSpecVersion("1.0")
+				.setType("org.springframework")
+				.putAttributes("name", CloudEventAttributeValue.newBuilder().setCeString("oleg").build())
+				.putAttributes("fluent_in_french", CloudEventAttributeValue.newBuilder().setCeBoolean(false).build())
+				.build();
+
+		ManagedChannel channel = ManagedChannelBuilder.forAddress("localhost", 6048)
+			.usePlaintext().build();
+
+		CloudEventServiceGrpc.CloudEventServiceBlockingStub stub = CloudEventServiceGrpc.newBlockingStub(channel);
+		CloudEvent reply = stub.requestReply(cloudEvent);
+		System.out.println(reply);
+
+	}
+
+	@Bean
+	public Function<Message<String>, Message<String>> uppercase() {
+		return message -> {
+			return MessageBuilder.withPayload(message.getPayload().toUpperCase())
+					.copyHeaders(message.getHeaders())
+					.setHeader("uppercased", "true")
+					.build();
+		};
+	}
+}
+
+