DataProxy: Populate X-Grafana-Referer header (#60040)

* ProxyUtil: Populate X-Grafana-Referer header

* ProxyUtil: Move Referer/Origin header removal

So that the removal and setting X-Grafana-Referer logic applies to all
proxied requests and not just datasource proxy.

* ProxyUtil: Test to guard against multiline headers

* ProxyUtil: Explicitly check injected header isn't parsed

pkg/util/proxyutil/reverse_proxy.go

@@ -76,10 +76,6 @@ func wrapDirector(d func(*http.Request)) func(req *http.Request) {
 
 		d(req)
 		PrepareProxyRequest(req)
-
-		// Clear Origin and Referer to avoid CORS issues
-		req.Header.Del("Origin")
-		req.Header.Del("Referer")
 	}
 }