feat(handlers): add support for tag concurrency to import API route

mentioned in commit e829b388

mentioned in issue #560 (closed)

mentioned in merge request gitlab-org/build/CNG!861 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1458 (merged)

mentioned in merge request gitlab-org/charts/gitlab!2346 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1463 (merged)

mentioned in merge request gitlab-development-kit!2385 (merged)

mentioned in merge request omnibus-gitlab!5899 (merged)