Add `create` cli command

One of the big themes from the Feedback issue: New Runner registration flow (gitlab#387993 - closed) is that we broke some automation and gitops scripts by requiring users to use the UI first.

1. We have a guide how to automate the runner creation https://docs.gitlab.com/ee/tutorials/automate_runner_creation/index.html
2. An idea to implement such logic in the helm chart Explore creating a Helm Chart to encapsulate th... (gitlab-org/charts/gitlab-runner#494 - closed)
3. Support in terraform provider

But all users still need to find a way implement it their own way.

Maybe we should add a command like gitlab-runner create --name "my runner" --tags --pat which would basically work as a workaround replacement for the old register command.

We can then simplify the docs.

An extreeme version of this idea: keep the register command, but teach it to work with PATs and basically behave the same way the old register command did 🤷