* Plugins: Add client middlware that forwards the signed grafana id token if present
* DsProxy: Set grafana id header if id token exists
* Add util function to apply id token to header
* Only add id forwarding middleware if feature toggle is enabled
* Add feature toggles to ds proxy and check if id forwarding is enabled
* Clean up test setup
* Change to use backend.ForwardHTTPHeaders interface
* PluginProxy: Forward signed identity when feature toggle is enabled
* PluginProxy: forrward signed id header