Provide on-demand compute #3

Currently, in order to use dstack, the user needs to either have an existing cloud account or own hardware.
It would be great if dstack provided its own compute provider and allow users to use dstack without having their own cloud account or hardware.
On one hand, dstack could provide a number of free GPU hours for the trial.
On the other hand, dstack could provide a way to pay for the spent hours, e.g. via a card.