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

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)