prevent config update at runtime except for plugins (#1955)

* prevent config update at runtime

They are not taken into account, except for plugins in adminspace.

* Notifier::insert_json5 returns an error if the provided key does not start with plugins/

---------

Co-authored-by: OlivierHecart <olivier.hecart@adlinktech.com>

Author: wyfo

zenoh/src/api/config.rs

@@ -248,6 +248,13 @@ impl Notifier<Config> {
     }
 
     pub fn insert_json5(&self, key: &str, value: &str) -> ZResult<()> {
+        if !key.starts_with("plugins/") {
+            bail!(
+                "Error inserting conf value {} : updating config is only \
+                    supported for keys starting with `plugins/`",
+                key
+            );
+        }
         self.lock_config().insert_json5(key, value)?;
         self.notify(key);
         Ok(())