Add `GetProjectMirrorPublicKey`, and add support for `auth_method` to `ProjectMirror`

mentioned in merge request gitlab-com/gl-infra/autolabels!22 (merged)

mentioned in merge request gitlab-org/cli!1976 (merged)

mentioned in merge request gitlab-org/terraform-provider-gitlab!2370 (merged)

mentioned in merge request gitlab-com/gl-infra/ringctl!248 (merged)

mentioned in merge request gitlab-com/gl-infra/platform/runway/runwayctl!879 (merged)

mentioned in merge request gitlab-org/release-tools!3982 (merged)

mentioned in merge request fforster/epic-update!2 (merged)

mentioned in merge request sue445/tanuki_reminder!229 (merged)

mentioned in merge request sue445/create-merge-request!111 (merged)

mentioned in merge request gitlab-com/gl-infra/oncall-robot-assistant!79 (merged)

mentioned in merge request ydkn/gitlab-runner-cleanup!35 (closed)

mentioned in merge request gitlab-org/container-registry!2154 (merged)

mentioned in merge request gitlab-com/content-sites/handbook-tools/hugolint!123 (merged)

mentioned in merge request !2265 (merged)