Add rhdh addon

[jasonmadigan]

What

Add an rhdh addon so oinc create --addons rhdh stands up Red Hat Developer Hub on the cluster, the same way the kuadrant addon works today. Consumers currently hand-roll this: kuadrant-backstage-plugin carries ~350 lines of helm mechanics and microshift workarounds in oinc/setup-rhdh.sh just to get stock RHDH running before layering its own plugins on top (see Kuadrant/kuadrant-backstage-plugin#348). The generic part belongs in oinc.

This issue is written as an implementation brief for an agent working in this repo.

Design

• Mirror the existing kuadrant addon structure and conventions. Do not invent a new addon mechanism.
• Version syntax: rhdh@<chart-version> pins the rhdh/backstage chart version from https://redhat-developer.github.io/rhdh-chart/. Note the chart carries no appVersion and is image-agnostic; rhdh@latest follows the chart index. Chart 6.2.2 pairs with the rhdh:1.10 image line (there is no 1.4 tag on quay, active minors are 1.5/1.8/1.9/1.10).
• Install stock RHDH into an rhdh namespace with guest auth enabled by default.
• Expose it properly: prefer a Route on the ports oinc already maps rather than leaving consumers to kubectl port-forward (a single forward dies on pod restart; consumers currently carry supervised reconnect loops to cope). Document the access URL either way.

Config hooks (what makes it useful for e2e consumers)

• Custom image override: repository, tag, and registry: \"\" so localhost/ sideloaded refs resolve, pullPolicy: IfNotPresent.
• A user values overlay file merged into the chart install, for dynamic-plugins config and app-config extras.
• Toggle to disable the quickstart onboarding overlay (its persistent progressbar breaks e2e page-ready waits).

Microshift quirks the addon must own (hard-won, do not rediscover)

• The chart default for dynamic-plugins-root is an ephemeral 5Gi PVC; microshift has no provisioner, so it must be overridden to emptyDir. Helm replaces the extraVolumes list wholesale: the override must re-declare the full seven-volume set the install-dynamic-plugins initContainer and main container mount (dynamic-plugins-root, dynamic-plugins, dynamic-plugins-npmrc, dynamic-plugins-registry-auth, npmcacache, extensions-catalog, temp), or the Deployment is rejected with orphan volumeMounts.
• Postgres: persistence off (emptyDir) with a sane ephemeral-storage limit.

A working reference for all of the above is oinc/setup-rhdh.sh on the PR branch linked overhead; absorb its generic parts, leave its kuadrant plugin config behind.

Acceptance criteria

• oinc create --version 4.21 --addons rhdh on a clean host yields a reachable stock RHDH (guest sign-in works)
• rhdh@<chart-version> pins the chart; rhdh@latest documented as following the chart index
• Image override and values overlay hooks work (proven by installing with a custom image ref and an overlay that adds a dynamic plugin)
• Composes with the kuadrant addon: --addons kuadrant@latest,rhdh on one cluster
• Works on a GitHub ubuntu-latest runner (smoke-tested in this repo's CI)
• README documents the addon, its options, and the access URL

Out of scope, related follow-ups

• oinc load-image <ref> subcommand (docker-archive into CRI-O via skopeo, the kind load docker-image equivalent)
• Pre-baking skopeo into the oinc image so consumers stop dnf installing it mid-job

Both are worth their own issues; kuadrant-backstage-plugin currently carries local versions of each.