Add notebook functions for starting dashboard and cloud functions

[nkitsaini]

No description provided.

[kesmit13]

I don't think that the notebook package is the right place for these functions. The notebook package is for interactive environment enhancements like surfacing magical objects that proxy a workspace group or workspace that is currently selected in an interactive notebook environment. I think the better place would be in functions or maybe create a new apps package at the top level. The Python UDF server helpers are in functions which is the closest thing to this functionality, but since dashboards aren't functions I could see that not being a great name. That would mean that the app_listen_port and app_url functions would move to that package as well.