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_test.go

@@ -50,6 +50,7 @@ func TestReverseProxy(t *testing.T) {
 		require.Equal(t, "10.0.0.1", actualReq.Header.Get("X-Forwarded-For"))
 		require.Empty(t, actualReq.Header.Get("Origin"))
 		require.Empty(t, actualReq.Header.Get("Referer"))
+		require.Equal(t, "https://test.com/api", actualReq.Header.Get("X-Grafana-Referer"))
 		require.Equal(t, "value", actualReq.Header.Get("X-KEY"))
 		resp := rec.Result()
 		require.Empty(t, resp.Cookies())