observability(runway): add artifact-registry hosted repositories dashboard

Adds dashboards/artifact-registry-gke/hosted.dashboard.jsonnet, a detail dashboard for the kind=hosted write, storage, and lifecycle paths: publishes per format, upload sessions and digest verification, payload sizes, deletion and purge, and the cross-cutting fault counters.

The layout is built to scale with formats: a top cross-format health row uses only format-comparable metrics (one panel per concern, formats as series), and each format's specific vocabulary lives in its own collapsed row, so a new format adds one row block instead of a panel per concern. Format-specific fault panels live with their format; the Faults row holds only format-neutral ones.

Like the other detail dashboards, it pins the Mimir - Runway datasource and replaces the stock environment template.

Rendered copy for review: https://dashboards.gitlab.net/d/jpereira-verify-ar-hosted