[Feature] WebUI: read-only access to selectors tab

[Feature] WebUI: read-only access to selectors tab

Open list_extractors, list_transforms and check_selector to
read-only users (enable = false, matching maps.lua). These only
return static catalogs or parse the selector without executing it.
check_message stays privileged (enable = true): it runs a
user-supplied selector against a user-supplied message, so it
remains denied to read-only users (HTTP 401)