Admin Token API: Identify Incoming Email Tokens

What does this MR do and why?

This MR adds support for identifying incoming email tokens to the Admin Token API.

with at Siemens

References

• Issue: #512033 (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 incoming mail token 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 an incoming email token of a user. If you've seeded the GDK, you can get a token from an existing user in the rails console:

User.last.incoming_email_token

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": "glimt-token-from-step-2.2"}'

{
	"id": 68,
	"username": "reported_user_22",
	"name": "Diann Turcotte",
	"state": "active",
	...
}

Related to #512033 (closed)

doc/api/admin/token.md

@@ -33,6 +33,7 @@ Prerequisites:
 > - [Pipeline trigger tokens added](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/174030) in GitLab 17.7.
 > - [CI/CD Job Tokens added](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/175234) in GitLab 17.9.
 > - [Feature flags client tokens added](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/177431) in GitLab 17.9.
+> - [Incoming email tokens added](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/177077) in GitLab 17.8.
 
 Gets information for a given token. This endpoint supports the following tokens:
 
@@ -46,6 +47,7 @@ Gets information for a given token. This endpoint supports the following tokens:
 - [Pipeline trigger tokens](../../ci/triggers/index.md#create-a-pipeline-trigger-token)
 - [CI/CD Job Tokens](../../security/tokens/index.md#cicd-job-tokens)
 - [Feature flags client tokens](../../operations/feature_flags.md#get-access-credentials)
+- [Incoming email tokens](../../security/tokens/index.md#incoming-email-token)
 
 ```plaintext
 POST /api/v4/admin/token