Admin Token API: Identify Feature Flags client token

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What does this MR do and why?

This MR adds identification of feature flags client token to the Admin Token API.

🛠️ with ❤️ at Siemens

References

• Issue: #512283 (closed)
• Epic: &15777
• Docs: https://docs.gitlab.com/ee/api/admin/token.html

MR acceptance checklist

checklist

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the javascript style guides
• Conforms to the database guides

How to set up and validate locally

1. You'll need to create a personal access token to access the token and get an feature flag client token that you'd like to query:

   1. Create a personal access token with admin_mode and api capabilities. Preferences > Access Tokens > Add a new token
   2. You'll need feature flag client token of a project. Access it from any project from Deploy > Feature flags > Configure. The Instance ID listed is the token. It should start with glffct.

2. Now you can retrieve information about this token:

curl -k --request POST \
--url 'https://gdk.test:3443/api/v4/admin/token' \
--header 'Authorization: Bearer <Admin Token from Step 2.1>' \
--header 'Content-Type: application/json' \
--data '{"token": "glffct-token-from-step-2.2"}'

{
	"id": 68,
	"project_id": 22
}

Related to #512283 (closed)