Add update workflow

[pitmonticone]

In brief, this workflow runs every 3 days at 8 AM UTC or can be triggered manually. It checks out the code, updates Lean project dependencies, and handles the outcome. If the update is successful, it creates a pull request (not direct push yet for precautionary reasons); if it fails, it opens an issue to notify us.

Of course let me know if you want to change anything about the scheduling.

[kbuzzard]

Thanks!