AWS cost attribution: automatically install infinity plugin and datasource for all our grafana instances

[consideRatio]

We currently have a step to install a plugin (infinity) and datasource (an infinity plugin based datasource). We could make this install by default in our grafana instances I think, and by doing so, remove a step from the documentation on enabling AWS cost attribution dashboards. Since we may want to do this for all AWS clusters, removing steps like these are relevant to not get bogged down with too many manual procedures.

Here is config of relevance to change for this to happen.

https://github.com/grafana/helm-charts/blob/5ed7405c446f13bbe004fef19a6ca53acda7dfb2/charts/grafana/values.yaml#L480-L505

Definition of done

• Configuring install of the inifinty plugin and a datasource is trialed and verified to work (or we come up with a new definition of done)
• Documentation about manually installing the inifinity plugin and datasource is cleaned up