[Improvement] Make engine startup user configurable at the `SERVER` share level

### 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.

### What would you like to be improved?

Make engine startup user configurable at the `SERVER` share level

### How should we improve?

_No response_

### Are you willing to submit PR?

- [ ] Yes. I can submit a PR independently to improve.
- [ ] Yes. I would be willing to submit a PR with guidance from the Kyuubi community to improve.
- [ ] No. I cannot submit a PR at this time.

Start by tracing the SERVER share-level configuration and the engine startup path in the Kyuubi codebase. Determine how startup users are currently selected and where the share-level setting should be read. Done means engine startup user selection is configurable at the SERVER share level, with behavior and tests covering the new setting.

Written by the indexing model from the issue text.