[telemetry] provide way to track the number of workspaces that are started with user devfiles (not pre-difined stacks)

[ibuziuk]

Description:
provide way to track the number of workspaces that are started with user devfiles (not pre-difined stacks):

Impl details
Add "custom" value if the stack is not defined.
A better idea would be also tracking the url parameter inside the events
probably instead of factory url we would track the devfile url.