Direct/Proxy mode switch

[jankowskimaciej]

How about adding an option to choose between Direct and Proxy mode?
As far as I know, current grafana-json-dataosource plugin supports only Proxy mode.
When all requests go through Grafana backend, it may generate too much load on the service.
Additionally, for some internal JSON APIs it's not possible to grant Grafana backend the access to read data.
Letting users fetch data directly from their browsers (with all required credentials) would solve that problem.