Cost Explorer API (10/n): adopt strategy for total cost per hub

[consideRatio]

Definition of done

• Let aws-ce-grafana-backend group total costs data into hub attributed costs (staging, prod, etc) and hub unattributed costs (shared).

  1. Filter by known 2i2c infra costs, and group by 2i2c:hub-name tag
     NOTE: We could also try group by kubernetes.io/created-for/pvc/namespace, but
     for now we avoid this complexity and treat all storage volumes as
     shared costs. Almost all storage costs stems from the prometheus server
     though, which lives in support namespace anyhow and not a hub
     specific namespace.
  2. Track the remaining hub unattributed costs separatly

• A manually defined grafana dashboard should be examplified to function against the backend. This should be panel 1 in [EPIC] Support attributing costs to individual hubs automatically on Openscapes #4453 (comment).
  

Practical notes

I've tested deployments of aws-ce-backend-grafana without using deployer deploy-support ... as that is very slow and complicated to iterate based on, but instead deployed this with helm directly into ce-test namespace in the openscapes cluster. Info about how to do that are in the readme files.