Schema Audit

I noticed some inconsistencies in our schemas

• Inconsistencies between attribute names and/or structures between the provider schemas and the upstream GitLab API schemas. (Example: gitlab_user attribute is_admin vs API field admin)
• Inconsistencies between attribute names across different resources in this provider, for attributes that have the same function. (Example: project vs id vs project_id vs full_path all being used to mean a project name/id)

Correcting inconsistencies will have these benefits

• More consistency between this provider and other API tools users may be using to manage GitLab.
• Increasing the number of resources that could be updated using code generation as the GitLab API evolves.

This will have to be a breaking change and a new major release.

However we could make some headway in a non-breaking way using state migrations