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
```

modules/setting/config_provider.go

@@ -26,6 +26,7 @@ type ConfigKey interface {
 	In(defaultVal string, candidates []string) string
 	String() string
 	Strings(delim string) []string
+	Bool() (bool, error)
 
 	MustString(defaultVal string) string
 	MustBool(defaultVal ...bool) bool