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/test/java/com/example/grpc/demo/DemoGrpcApplicationTests.java

@@ -0,0 +1,13 @@
+package com.example.grpc.demo;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.boot.test.context.SpringBootTest;
+
+@SpringBootTest
+class DemoGrpcApplicationTests {
+
+	@Test
+	void contextLoads() {
+	}
+
+}