Add support for functions that return a Future

Add support for functions to run async. For example

```java
@FunctionName("HttpTrigger-JavaAsync")
public CompletableFuture runAsync(
@HttpTrigger(name = "req", methods = {HttpMethod.GET, HttpMethod.POST}, authLevel = AuthorizationLevel.FUNCTION) HttpRequestMessage> request,
final ExecutionContext context)
```

Java worker should inspect function method signature, if it returns a `Future` invoke it asynchronously
and if not invoke the method synchronously.