Move java samples to the java invoker

and update for new inovker model

Author: scothis

samples/greeter/.gitignore

@@ -0,0 +1,6 @@
+/*
+!.gitignore
+!src
+!mvnw*
+!pom.xml
+!README.*

samples/greeter/README.adoc

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+= greeter
+
+[horizontal]
+*Invoker*:: java
+*Function*:: link:src/main/java/functions/Greeter.java[Greeter.java]
+*Input*:: names
+
+=== prerequisites
+
+1. riff is deployed
+2. the `riff` CLI is on your PATH
+3. the java invoker is installed
+
+    riff invokers apply -f https://github.com/projectriff/java-function-invoker/raw/master/java-invoker.yaml
+
+4. the working directory is `samples/greeter`
+
+=== compile the Java code and build the JAR
+
+```
+./mvnw clean package
+```
+
+=== create the function and its input topic
+
+```
+riff create java --input names --handler functions.Greeter
+```
+
+=== publish a message and wait for a reply
+
+```
+riff publish --input names --data World --reply
+```
+
+=== delete the function and its input topic
+
+```
+riff delete --all
+```