add post import API endpoint

lib/api/project_import.rb

+module API
+  class ProjectImport < Grape::API
+    include PaginationParams
+
+    helpers do
+      def import_params
+        declared_params(include_missing: false)
+      end
+
+      def file_is_valid?
+        import_params[:file] && import_params[:file].respond_to?(:read)
+      end
+    end
+
+    before do
+      not_found! unless Gitlab::CurrentSettings.import_sources.include?('gitlab_project')
+    end
+
+    params do
+      optional :namespace, type: String, desc: 'The ID or name of the namespace that the project will be imported into. Defaults to the user namespace.'
+      requires :file, type: File, desc: 'The project export file to be imported'
+    end
+    resource :projects do
+      desc 'Get export status' do
+        success Entities::ProjectImportStatus
+      end
+      post 'import' do
+        render_api_error!('The branch refname is invalid', 400) unless file_is_valid?
+
+        namespace = import_params[:namespace]
+
+        namespace = if namespace && namespace =~ /^\d+$/
+                      Namespace.find_by(id: namespace)
+                    elsif namespace.blank?
+                      current_user.namespace
+                    else
+                      Namespace.find_by_path_or_name(namespace)
+                    end
+
+        project = ::Projects::GitlabProjectsImportService.new(current_user, import_params).execute
+
+        render_api_error!(link.project.full_messages.first, 400) unless project.saved?
+
+        present project, with: Entities::ProjectImportStatus
+      end
+    end
+  end
+end