Plugins: Forward user header (X-Grafana-User) in backend plugin requests (#58646)

Grafana would forward the X-Grafana-User header to backend plugin request when 
dataproxy.send_user_header is enabled. In addition, X-Grafana-User will be automatically
forwarded in outgoing HTTP requests for core/builtin HTTP datasources. 
Use grafana-plugin-sdk-go v0.147.0.

Fixes #47734

Co-authored-by: Will Browne <wbrowne@users.noreply.github.com>

pkg/api/pluginproxy/utils.go

@@ -9,7 +9,6 @@ import (
 	"text/template"
 
 	"github.com/grafana/grafana/pkg/plugins"
-	"github.com/grafana/grafana/pkg/services/user"
 )
 
 // interpolateString accepts template data and return a string with substitutions
@@ -84,11 +83,3 @@ func setBodyContent(req *http.Request, route *plugins.Route, data templateData)
 
 	return nil
 }
-
-// Set the X-Grafana-User header if needed (and remove if not)
-func applyUserHeader(sendUserHeader bool, req *http.Request, user *user.SignedInUser) {
-	req.Header.Del("X-Grafana-User")
-	if sendUserHeader && !user.IsAnonymous {
-		req.Header.Set("X-Grafana-User", user.Login)
-	}
-}