Notebook kernel source menu contribution

[rebornix]

This PR is an API experiment for #146942 based on offline discussions with @jrieken . The idea is allowing extensions to contribute commands to the kernel picker list through menu contribution. This gives extensions the full capabilities of implementing proxy kernels, kernel servers, etc. The menu contribution can be replaced by real APIs if necessary (especially if we have quite some interops with other models in the notebook).

The code in this PR is large, but it's basically:

• Revert changes in Experiment: proxy controller in notebooks #147470, no ProxyKernel concept in the core or API.
• Adds a menu contribution notebook/kernelSource
• We will render both kernels/controllers and commands in notebook/kernelSource

In addition to above, here are two typical workflows enabled:

Single source command

When there is only one single extension contributing a command to menu notebook/kernelSource, users will see that command in the kernel status bar item, as shown below

When users attempt to run cells, the core will run the command. When the command finishes execution (promise resolves), we will detect if there are notebook controllers created for current notebook, if there is a preferred one, pass the pending cell execution request to the controller, finish the cell execution.

Mixed notebook kernels and kernel source commands

When we have multiple kernels and commands contributed by extensions, the kernel status bar item behaves the same as before, it will show the suggested/preferred kernel name if there is any.

When users click the kernel status item, we will show the kernel picker, which lists all available kernels and commands contributed to menu notebook/kernelSource. If users pick a command/action from notebook/kernelSource, the core will run the command. At the end, we might get more kernels/controllers contributed as a side effect of running the command. Users can then run cells as usual.

[jrieken]

Looking good.Left a nit-comment. CI is failing, no approval because of that

[roblourens]

Sweet