Add RHUI support for building images on cloud instances

[brandonrc]

Summary

The images library has no support for RHUI (Red Hat Update Infrastructure) repos. Cloud Access and on-demand RHEL instances use RHUI for package updates, but RepoConfig only has an RHSM flag — there is no RHUI equivalent. This means the depsolve pipeline cannot distinguish RHUI repos from RHSM repos, and NewCurlPackageItem() cannot generate manifests with org.osbuild.rhui secrets.

This was originally reported in osbuild/osbuild-composer#820 and closed as stale.

Problem

1. No RHUI field on RepoConfig — repos cannot be flagged as using cloud instance identity authentication
2. NewCurlPackageItem() missing RHUI case — the secrets switch only handles org.osbuild.rhsm and org.osbuild.mtls, dropping RHUI secrets silently
3. applyRHSMSecrets() only handles RHSM — packages from RHUI repos don't get the correct secrets provider
4. validateSubscriptionsForRepos() blocks RHUI repos — RHUI repos have RHSM=true but no entitlement certs, causing a subscription validation error

Proposed Solution

1. Add RHUI bool field to RepoConfig and repository structs
2. Add case "org.osbuild.rhui" to NewCurlPackageItem()
3. Update applyRHSMSecrets() to set org.osbuild.rhui on RHUI packages
4. Update validateSubscriptionsForRepos() to skip RHUI repos
5. Pass RHUI flag through v2Repository to the Python depsolve solver

Stopgap

Until native support is merged, users can use osbuild-rhui-shim as a workaround.

References

• Unable to build images on AWS Cloud Access images that use RHUI osbuild-composer#820 — original report (closed as stale)
• Add native RHUI support for building images on cloud instances osbuild#2354 — companion osbuild issue
• Add RHUI support for building images on cloud instances osbuild-composer#5028 — companion osbuild-composer issue