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/pluginproxy.go

@@ -154,7 +154,7 @@ func (proxy PluginProxy) director(req *http.Request) {
 
 	req.Header.Set("X-Grafana-Context", string(ctxJSON))
 
-	applyUserHeader(proxy.cfg.SendUserHeader, req, proxy.ctx.SignedInUser)
+	proxyutil.ApplyUserHeader(proxy.cfg.SendUserHeader, req, proxy.ctx.SignedInUser)
 
 	if err := addHeaders(&req.Header, proxy.matchedRoute, data); err != nil {
 		proxy.ctx.JsonApiErr(500, "Failed to render plugin headers", err)