Fix config's handling of non-ascii characters - #41657

Summary of the Pull Request

When parsing the config file, non-ascii utf-16 values are casted into char. This will mangle non-ascii values in the config file. For example, a custom kernel path containing Chinese characters will cause WSL_E_CUSTOM_KERNEL_NOT_FOUND.

This PR removes the faulty static cast and stores the config value in wstring instead of string. So non-ascii values are preserved.

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
• 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

Modify test:
UnitTests::UnitTets::WriteWslConfig

Manually tested with a custom kernel with a path containing Chinese characters. Before the fix, wsl fails with:

The custom kernel specified in C:\Users\wangfen\.wslconfig was not found: 'D:\-��8\kernel'.
Error code: Wsl/Service/CreateInstance/CreateVm/WSL_E_CUSTOM_KERNEL_NOT_FOUND
If this error was unexpected, please consider searching for existing issues or filing a new issue at https://github.com/microsoft/WSL/issues.

After the fix, wsl boots correctly.