Backend API to create Service Account User Self Managed

For Service Account Users

&6777

We are already done with Backend Service for creating users

This Issue for creating an API creating users of type Service Accounts. We may need to make changes to POST API call for users to handle user of type Service Accounts

Following API call is there for admins only and can be used for Self-managed

https://docs.gitlab.com/ee/api/users.html#user-creation