CurrentUserContext to carry current user identity to the authentication layer

[kostrse]

The final goal is to completely separate authentication logic from the business layer. The token request logic should not live alongside with normal query code. This will allow to abstract out authentication from the core plugin implementation and eventually move authentication to the Grafana Azure SDK.

This is intermediate PR which introduces CurrentUserContext context abstraction which is supposed to carry current user identity though context down to the transport layer.

ADX client extracts the CurrentUserContext and uses it for authentication regardless of where the context was originated.

Helper methods adxusercontext.FromQueryReq and adxusercontext.FromResourceReq provided to create user context for query (here) and resource (here) requests. After the context was created, it can be passed to the rest of the business logic, down to ADX client without having to have any handling of access token or authorization header.

Problem with heath check

Due to current design of Plugin SDK, CheckHealth doesn't carry ID token (see here), so this PR has a workaround specifically for TestRequest to use service identity via GetServiceAccessToken(ctx) for datasource testing (here), rather than using the proper user identity.

This is incorrect behavior which already existed before this PR. This PR just isolated it to CheckHealth/TestRequest so the issue becomes more straightforward to address.

• All requests should use user identity auth if user identity configured #509
• IdToken as part of PluginContext.User grafana-plugin-sdk-go#579

[andresmgot]

I am okay with the main change, my main concern is that we have both the context and the backend.Requests in the different methods and we are choosing to add the token info to the context rather than the request (which is what I would thought include the auth info at first glance) but it's probably not a big issue if we are explicit in the code about the decission.

Also, there is also the potential problem that the tokens may be pass down to functions that should not have access to them (but that have access to the context) but at the moment I don't see any example. What do you think?

[kostrse]

I created a separate issue to remove remaining headers out of the modelQuery, it should not need backend.User anymore:

• Move usage tracking from business layer to ADX client #526

[andresmgot]

There is one comment left and some spell / lint errors, let me know if you need any help with that

[kostrse]

There is one comment left and some spell / lint errors, let me know if you need any help with that

Tests fixed.

[andresmgot]

LGTM, thanks for the changes!