Introduce Vue UI to the runner details [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This MR introduces Vue for first time to the Runner details page 🎉

The Vue implementation only contains the page heading as a first "invisible" iteration, more of the page is still rendered in HAML, and as we progress, more of the page should be implemented in Vue.

In particular this MR:

• Adds a new entry point for admin runners ../admin/runners/show/index.js with a corresponding file to boot Vue.
• Adds a base runner_detail_app.vue that we will build upon, this component is very simple now, but it will eventually hold the logic for the entire Runner details.
• Does everything from a feature flag, so we can start expending the scope of the app that is built in Vue.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this change should not impact user facing features.
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #300507 (closed)