Add a default sever url to the it the serverUrl field
Default url should be:
https://gitlab.com
User may want to change it to other server urls like:
http://gitlab.example.com:7990 https://gitlab.example.com
In the doCheckServerUrl method, open up the serverUrl and see if it is a valid serverUrl. See how it this part is gone in the Gitea Plugin and the GitHub Plugin.