feat(stepfunctions-tasks): support FLEX execution class for GlueStartJobRun (#30534)

### Issue # (if applicable)

Closes #30533 

### Reason for this change
To support `FLEX` execution class for `GlusStartJobRun`.
By using `FLEX` class, you can reduce the cost of running Glue job.



### Description of changes
Add `execution class` to the `GlusStartJobRun` class.


### Description of how you validated changes
Add unit tests and integ tests.


### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: mazyu36