Allow for other kernel_spec_manager classes

[ClaytonAstrom]

Problem

kernel_spec_manager is hardcoded to a jupyter_client KernelSpecManager instance, not allowing for async or other managers - https://github.com/voila-dashboards/voila/blob/main/voila/app.py#L547

Proposed Solution

• Expose the top level kernel_spec_manager to the VoilaConfiguration

[martinRenou]

Thank you for opening an issue! Making this configurable could be nice indeed. Would you be willing to try opening a PR for this?