Refactor the session creation logic to match the new API

[OneBlue]

Summary of the Pull Request

** This change depends on: https://github.com/microsoft/WSL/pull/13791 **

This change refactors the session creation logic to:

• Make the storage disk creation easier
• Get rid of the old VM settings
• Run the init script from within the service
• Simplify testing

PR Checklist

• [ ] Closes: Link to issue #xxx
• [ ] Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• [X] Tests: Added/updated if needed and all pass
• [ ] Localization: All end user facing strings can be localized
• [ ] Dev docs: Added/updated if needed
• [ ] Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed