[Improvement][rust] Set the default value for config key in toml

### 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-uniffle/issues?q=is%3Aissue) and found no similar issues.

### What would you like to be improved?

Currently, the optional value of conf key is scattered everywhere, we should set the default value if it is not shown in toml config file.

Refer: https://github.com/serde-rs/serde/issues/725

### How should we improve?

_No response_

### Are you willing to submit PR?

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