Add user to CompletionRequest (#23)

Fixes https://github.com/TheoKanning/openai-java/issues/19

api/src/main/java/com/theokanning/openai/completion/CompletionRequest.java

@@ -108,4 +108,9 @@ public class CompletionRequest {
      * best_of must be greater than n.
      */
     Integer bestOf;
+
+    /**
+     * A unique identifier representing your end-user, which will help OpenAI to monitor and detect abuse.
+     */
+    String user;
 }

client/src/test/java/com/theokanning/openai/CompletionTest.java

@@ -19,6 +19,7 @@ public class CompletionTest {
         CompletionRequest completionRequest = CompletionRequest.builder()
                 .prompt("Somebody once told me the world is gonna roll me")
                 .echo(true)
+                .user("testing")
                 .build();
 
         List<CompletionChoice> choices = service.createCompletion("ada", completionRequest).getChoices();

example/src/main/java/example/OpenAiApiExample.java

@@ -23,6 +23,7 @@ class OpenAiApiExample {
         CompletionRequest completionRequest = CompletionRequest.builder()
                 .prompt("Somebody once told me the world is gonna roll me")
                 .echo(true)
+                .user("testing")
                 .build();
         service.createCompletion("ada", completionRequest).getChoices().forEach(System.out::println);