feat(config): implement extensions API

mentioned in commit 533d9211

mentioned in merge request gitlab-org/cli!2249 (merged)

mentioned in merge request gitlab-org/terraform-provider-gitlab!2589 (merged)

mentioned in merge request gitlab-com/gl-infra/platform/runway/runwayctl!1086 (merged)

mentioned in merge request gitlab-org/cli!2250 (closed)

mentioned in merge request gitlab-org/release-tools!4331 (merged)

mentioned in merge request gitlab-com/gl-infra/woodhouse!667 (merged)

mentioned in merge request fforster/gitlab-mcp!130 (merged)

mentioned in merge request schober-server/kubernetes/images/gitlab-token-refresher!21 (merged)

mentioned in merge request gitlab-com/gl-infra/autolabels!73 (merged)

mentioned in merge request gitlab-com/gl-infra/ringctl!325 (merged)

mentioned in merge request gitlab-org/container-registry!2420 (merged)

mentioned in merge request sue445/tanuki_reminder!254 (merged)

mentioned in merge request sue445/create-merge-request!138 (merged)

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

mentioned in merge request fforster/epic-update!14 (merged)