Cache xpkg.upbound.io registry

[IvanJosipovic]

What is the problem you're trying to solve
Unable to add xpkg.upbound.io to the ACR Cache Rules.

"Unsupported upstream or login server 'xpkg.upbound.io/*' provided. Supported upstreams or login servers are: docker.io, mcr.microsoft.com, quay.io, public.ecr.aws, ghcr.io, nvcr.io, registry.k8s.io, gcr.io, eu.gcr.io."

Describe the solution you'd like
The ability to use ACR Cache with the xpkg.upbound.io repository.

Additional context
#686