Specify additional headers for externally managed Grafana

[Dermah]

We host our Grafana instance behind a proxy that requires a few authentication headers, such as:

Client-Id: <CLIENT_ID>
Client-Secret: <CLIENT_SECRET>

We would like to use the grafana-operator, running in a different network, to connect to this Grafana instance and manage it as an external Grafana. However it appears the external spec only allows for usernames, passwords and api keys.

It would be great for us if there was a field on Grafana.spec.external such as additional_headers (as originally proposed in #402 (comment)) that could be populated with arbitrary headers that are used for all requests to the external Grafana.

Perhaps it would also be good to have these headers set from a ConfigMap or Secret

[theSuess]

Thanks for the request! Can you put together an example of how you'd like this to look in the Grafana resource? Just so we don't implement this and then miss your use case :)