admin: Deploy redis instance for sccache

We would like to deploy sccache as a cache for compiled rust artifacts on jenkins. This cache will be used for swh-graph and swh-provenance CI, and to speed up image builds for containers including rust tooling such as rage and swh-graph.

To leverage cached objects across jobs, we need to deploy a centralized storage backend for sccache (the sccache server isn't meant to be deployed as a persistent service). It seems that the most appropriate candidate to do so is redis, as we already have manifests to deploy redis and we should be able to give it fast storage.

The cache storage doesn't really need persistence (as long as it's not gratuitously flushed) so it would make sense to deploy it on faster scratch storage.

TODO list:

• add scratch storage to admin k8s nodes scratch storage is already configured
• deploy local-path-provisioner on admin cluster
• update cluster-components redis chart to support authentication and external service
• deploy redis for sccache on admin cluster
• open appropriate firewall ports for access from jenkins nodes to the sccache redis cluster