Add Runner registration token blueprint

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR adds a blueprint of the MVC for removing Runner registration tokens. It is meant as a venue for discussion, but may eventually be merged after everyone has had a chance to contribute to the discussion.

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MR acceptance checklist

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Part of https://gitlab.com/gitlab-org/gitlab-runner/-/issues/28915, https://gitlab.com/gitlab-org/architecture/tasks/-/issues/27