Support attribute for default process mode of projects once it's available

Once Allow adjusting the default process_mode for re... (gitlab#378628 - closed) is implemented we want to support that new default process mode attribute in the Terraform provider - that is, in the gitlab_project resource and related data sources.

resource "gitlab_project" "main" {
    project              = "namespace/myproject"
    default_process_mode = "newest_first"
}

Related GitLab APIs

API documentation: not yet available

Additional Details

• GitLab REST API resources available (read for data sources, crud for resources)
• go-gitlab does support the related GitLab API already
• I'd like to contribute it myself