Add `extension-refresh` feature flag (#5106)

- in case people want to try out the new extension design

src/config.rs

@@ -812,7 +812,7 @@ fn validate_config(cfg: &ConfigItems) -> Result<(), Error> {
 
     // TODO: deal with deprecated flags so they can be removed from this list, cf. #4263
     const KNOWN_FLAGS: &[&str] =
-        &["autofill-overlay", "autofill-v2", "browser-fileless-import", "fido2-vault-credentials"];
+        &["autofill-overlay", "autofill-v2", "browser-fileless-import", "extension-refresh", "fido2-vault-credentials"];
     let configured_flags = parse_experimental_client_feature_flags(&cfg.experimental_client_feature_flags);
     let invalid_flags: Vec<_> = configured_flags.keys().filter(|flag| !KNOWN_FLAGS.contains(&flag.as_str())).collect();
     if !invalid_flags.is_empty() {