Add support for providing just an endpoint to hit for data, instead of proxying/port-forwarding into a cluster

[michaelmdresser]

Some users may wish to run kubectl cost against an already-exposed endpoint (if Kubecost endpoints are exposed publicly) or a local version of OpenCost during development.

We could add support for this with a third query mode, kubectl cost --no-cluster --path "kubecost.example.com" (or similar) that just uses an API endpoint path to make queries and doesn't port-forward or proxy using a K8s client. Flag names TBD.

This is currently low priority. If you are interested in this functionality, please comment on this issue!

[michaelmdresser]

• Closing in favor of more detailed issue: Introduce a pure-HTTP (no K8s client) query mode for use when only API access is available (e.g. Kubecost Cloud) #128