datasource/gitlab_projects: Allow to get archived and unarchived repositories. Closes #452

This change makes use of the deprecated d.GetOkExists() function to check if a boolean attribute has been set by the user or not. AFAIK, it's the only way to achieve that in the SDK v2. It doesn't seem to go anywhere without a proper replacement.

See:

• https://github.com/hashicorp/terraform-plugin-sdk/issues/817
• https://github.com/hashicorp/terraform-plugin-sdk/pull/350#issuecomment-597888969

Closes #452 (closed)