Add --help and --show-progress opts to gdk start

What does this Merge Request do and why?

This MR adds --help and --show-progress options when running gdk start to make it easier and more obvious how long it takes to start up a GDK instance:

$ gdk start --help
Usage: gdk start [<args>]

  --help             Display help
  --show-progress   Once started, time how long to boot

$ gdk start --show-progress
ok: run: ./services/gitlab-workhorse: (pid 275) 0s, normally down
ok: run: ./services/postgresql: (pid 276) 0s, normally down
ok: run: ./services/praefect: (pid 277) 0s, normally down
ok: run: ./services/praefect-gitaly-0: (pid 279) 0s, normally down
ok: run: ./services/rails-background-jobs: (pid 278) 0s, normally down
ok: run: ./services/rails-web: (pid 280) 0s, normally down
ok: run: ./services/redis: (pid 283) 0s, normally down
ok: run: ./services/sshd: (pid 281) 0s, normally down
ok: run: ./services/webpack: (pid 282) 0s, normally down

=> GitLab will be available at http://127.0.0.1:3001 shortly.
=> Waiting until http://127.0.0.1:3001 is ready..........
=> http://127.0.0.1:3001/users/sign_in is up (200 OK). Took 65.54 second(s).

Of course support/test_url still works the same, but is now Ruby and uses the same code as gdk start --show-progress 🙂

$ gdk start ; support/test_url
ok: run: ./services/gitlab-workhorse: (pid 1890) 1s, normally down
ok: run: ./services/postgresql: (pid 1891) 1s, normally down
ok: run: ./services/praefect-gitaly-0: (pid 1892) 1s, normally down
ok: run: ./services/rails-background-jobs: (pid 1894) 1s, normally down
ok: run: ./services/rails-web: (pid 1895) 1s, normally down
ok: run: ./services/redis: (pid 1896) 1s, normally down
ok: run: ./services/sshd: (pid 1897) 1s, normally down
ok: run: ./services/webpack: (pid 1898) 1s, normally down
ok: run: ./services/praefect: (pid 2465) 0s, normally down

=> GitLab will be available at http://127.0.0.1:3001 shortly.
.......
=> http://127.0.0.1:3001/users/sign_in is up (200 OK). Took 60.58 second(s).

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.