All requests should use user identity auth if user identity configured

[kostrse]

When user configures OBO authentication then some requests authenticated using the user identity of OBO and some continue to use service identity directly.

Properly using user identity:

• QueryData (here)

Methods using service identity directly (need to be fixed):

• CheckHealth(here)
• CallResource getDatabases (here)
• CallResource getSchema (here)

Regardless of type of the request, authentication method should be consistent with user's configuration.

[kostrse]

This PR #525 isolates the issue to CheckHealth (here).

Short term solution for CheckHealth would be to temporary remove server-side implementation and implement it via testDatasource() on frontend which would make a normal query request.

Ideal solution would be grafana/grafana-plugin-sdk-go#579 to include IdToken to req.PluginContext.User (here), so ID token could be accessed from the User struct regardless of which backend func was called (query req, resource call, etc.).