Validate the provided username and return an authentication token if valid.
curl --request POST \ --url https://v3.minestorecms.com/api/auth/username \ --header 'Content-Type: application/json' \ --data '{ "username": "xMarkus" }'
"eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ94343"
OK
The authentication token (40 digits).