feat(config): support custom headers for instances

mentioned in commit 5fe68f2a

mentioned in merge request gitlab-org/cli!2314 (merged)

mentioned in merge request gitlab-com/gl-infra/platform/runway/runwayctl!1141 (merged)

mentioned in merge request gitlab-com/gl-infra/woodhouse!681 (merged)

mentioned in merge request gitlab-org/release-tools!4401 (merged)

mentioned in merge request fforster/gitlab-mcp!150 (merged)

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

mentioned in merge request gitlab-com/gl-infra/autolabels!86 (merged)

mentioned in merge request gitlab-com/gl-infra/ringctl!339 (merged)

mentioned in merge request gitlab-org/terraform-provider-gitlab!2631 (merged)

mentioned in merge request sue445/tanuki_reminder!260 (merged)

mentioned in merge request sue445/create-merge-request!146 (merged)

mentioned in merge request hojerdev/git-sync!64 (merged)

mentioned in merge request gitlab-org/container-registry!2558 (merged)