Add `-w, --watch` argument to the `dstack ps` command

[peterschmidt85]

Sometimes, the user may want to watch the progress of running workflows in real-time.

Currently, the easiest way to do it is by invoking the dstack ps command. However, in order to the status in real-time, the user either have to invoke it manually every once in a while, or use the command watch dstack ps. The latter solves the problem but doesn't respect the colored output of the dstack ps command.

Let's implement -w (--watch argument) in the dstack ps command to show the status in real-time, e.g. every 2 seconds, until the user doesn't interrupt the command.