Add Artifact Registry settings to the GitLab Helm chart appConfig

What

Chart support for configuring GitLab's connection to the Artifact Registry service: a global.appConfig.artifactRegistry block that renders the artifact_registry: section into gitlab.yml and mounts the service-token secret.

Why

Rails already has artifact_registry.api_url (defaults only in dev/test), and #617724 adds the service-token setting for the internal API. The chart exposes none of it: no artifactRegistry appConfig, no gitlab.yml rendering, no secret mount. Any Kubernetes install (including the gitlab-caproni dev rig) has to inject raw gitlab.yml content or env overrides to point GitLab at Artifact Registry.

Shape

Mirror charts/gitlab!5125 (iam_data_access_service: appConfig block, gitlab.yml template gated so installs that leave it unset render nothing, secret mounts, doc page, config spec), adapted to an HTTP service: api_url + authToken.{secret,key} instead of grpc.host/port.

Out of scope

• The Rails settings and credential provider (#617724).
• Minting and distributing the secret.