[FEATURE] Add realUser field for KyuubiSession to identify realUser and proxyUser

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

### Search before asking

- [X] I have searched in the [issues](https://github.com/apache/incubator-kyuubi/issues?q=is%3Aissue) and found no similar issues.

### Describe the feature

Return the realUser and proxyUser option by getUserName method.
```
def getUserName(sessionConf: Map[String, String]): (String, Option[String])
```

### Motivation

To show more useful info, especially for batch restful api.

### Describe the solution

_No response_

### Additional context

_No response_

### Are you willing to submit PR?

- [x] Yes I am willing to submit a PR!