Add ability to authenticate to grafana using AAD auth.

[KnicKnic]

Addresses #192

1. Adds USE_AAD_AUTH to use aad for grafana.
2. Adds to GrafanaConfig a AAD default credential if USE_AAD_AUTH is set.
3. Changed storage of GrafanaClient to a functor to allow computation of the grafana client
   1. This seemed like the cleanest way to update the code
   2. Updated GrafanaClient functor to allow returning errors and fixed up all call sites to deal with errors.
4. updated the promClientFromContext to assume it is goign to a Azure Monitor workspace and login again with AAD auth
   1. I didn't add logic to cache this client as it isn't being cached today, but it is a potential source of improvement.

[CLAassistant]

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