AWS cost attribution (2c.2/4): [Spike 2h] Figure out how to define an infinity query variable

[GeorgianaElena]

See #4934 (comment) for context

Definition of Done

• there are panels of cost per component per hub correctly being rendered
• there is a variable defined in grafonnet, similar to the one created by hand in the openscapes grafana

[consideRatio]

As part of going for this, I went for jupyterhub/grafana-dashboards#142 as the grafonnet docs are only available for that version, and I also didn't want to invest time exploring whats available in an older version when it could boil down to needing the new version.

[consideRatio]

For things to work properly, we actually needed grafonnet v11.1.0, so I figure I want to verify things work after we have jupyterhub/grafana-dashboards#142 merged. It worked when I did things locally though.

[consideRatio]

Verified success!