Inspect FunctionType to extract collection item type

Resurrects some code from #222 (the test didn't go far enough
when that issue was closed).

spring-cloud-function-samples/function-sample-aws/pom.xml

@@ -14,7 +14,7 @@
 	<parent>
 		<groupId>org.springframework.boot</groupId>
 		<artifactId>spring-boot-starter-parent</artifactId>
-		<version>2.1.0.BUILD-SNAPSHOT</version>
+		<version>2.1.0.RC1</version>
 		<relativePath /> <!-- lookup parent from repository -->
 	</parent>