[Feature Request] exit message

**FEATURE REQUEST**

It would be nice if argo supported workflow exit message (output message / parameters?) in addition to exit status. This exit message should be directly available when getting workflow result:
```
argo get
````

This is useful when result of workflow is short text, such as container tag (in case of CI workflow).

While it is possible to use exit container that runs "echo" for given message, such solution is not optimal for scenario where simple exit message is needed, as it requires first getting workflow result, and then container logs from given container.

Start at the `argo get ` entry point and trace how workflow results are represented and displayed. Determine where an exit message or output parameter would be exposed, then verify that a short workflow result is shown directly by `argo get` without requiring an exit-container log lookup.

Written by the indexing model from the issue text.