feat: add WithUserAgent client option

mentioned in commit eef27017

mentioned in merge request gitlab-com/gl-infra/ringctl!320 (merged)

mentioned in merge request gitlab-org/cli!2201 (merged)

mentioned in merge request gitlab-org/terraform-provider-gitlab!2565 (merged)

mentioned in merge request gitlab-com/gl-infra/autolabels!66 (merged)

mentioned in merge request gitlab-com/gl-infra/woodhouse!659 (merged)

mentioned in merge request gitlab-org/release-tools!4295 (merged)

mentioned in merge request fforster/gitlab-mcp!104 (merged)

mentioned in merge request schober-server/kubernetes/images/gitlab-token-refresher!18 (merged)

mentioned in merge request gitlab-com/gl-infra/platform/runway/runwayctl!1053 (merged)

mentioned in merge request gitlab-org/container-registry!2376 (merged)

mentioned in merge request fforster/epic-update!12 (merged)

mentioned in merge request gitlab-com/gl-infra/oncall-robot-assistant!104