Add valkey-operator artifactapi remotes

Adds a valkey helm remote and allowlists the operator/exporter images so the
valkey-operator install in argocd-apps resolves everything through artifactapi.

- Add config/remote_helm/valkey.yaml proxying https://valkey.io/valkey-helm
- Add valkey to the virtual/helm member list
- Allow ^valkey-io/ on the ghcr remote (operator image ghcr.io/valkey-io/valkey-operator)
- Allow ^oliver006/redis_exporter on dockerhub (Valkey metrics exporter)

config/remote_helm/valkey.yaml

@@ -0,0 +1,7 @@
+base_url: https://valkey.io/valkey-helm
+description: Valkey Helm charts (valkey-operator)
+immutable_ttl: 0
+mutable_ttl: 3600
+check_mutable: true
+immutable_patterns:
+  - "\\.tgz$"