Notebook cell execution model

[jrieken]

Somewhat a continuation of #103713

Today, execution results are parts metadata and output and execution itself is done via kernels which come from kernel providers. We should try to model execution results into a single entity, e.g execution status (idle, running, scheduled? etc)

[jrieken]

One thing that we should do is to allow the executeCell-functions to return a promise. The promise should resolve that execution has been successfully started. It would allow to show feedback on the run button in cases in which the kernel needs to be resolved or isn't idle.