PTFM-10902: dx/dxpy support for job priority

- dx run --priority {normal,high} to request a particular priority
- Priority defaults to "high" when run with "dx run --watch" or when run
  immediately when building an executable ("dx build --run")
- dx prints a warning when normal priority is requested, but the
  executable will be requesting extra permissions (Internet access,
  developer access, or write access to projects)