One-line GDK install script

What does this Merge Request do and why?

This MR offers the ability to install a GDK from scratch with one command:

$ curl https://gitlab.com/gitlab-org/gitlab-development-kit/-/raw/main/support/install | bash -

Right now, to test you'll need to use:

curl "https://gitlab.com/gitlab-org/gitlab-development-kit/-/raw/e853a41f43fb9542976e8d0f93cdb6a502f7a356/support/install" | bash -

Tested working on:

• macOS Big Sur
• Ubuntu 20.04.2 LTs

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.

Closes #833 (closed)