Add a config option to block "expensive" pages for anonymous users (#34024)

Fix #33966

```
;; User must sign in to view anything.
;; It could be set to "expensive" to block anonymous users accessing some pages which consume a lot of resources,
;; for example: block anonymous AI crawlers from accessing repo code pages.
;; The "expensive" mode is experimental and subject to change.
;REQUIRE_SIGNIN_VIEW = false
```

routers/private/serv.go

@@ -286,7 +286,7 @@ func ServCommand(ctx *context.PrivateContext) {
 			repo.IsPrivate ||
 			owner.Visibility.IsPrivate() ||
 			(user != nil && user.IsRestricted) || // user will be nil if the key is a deploykey
-			setting.Service.RequireSignInView) {
+			setting.Service.RequireSignInViewStrict) {
 		if key.Type == asymkey_model.KeyTypeDeploy {
 			if deployKey.Mode < mode {
 				ctx.JSON(http.StatusUnauthorized, private.Response{