[Bug] Undefined Behavior in `zenoh_config::Notifier`

[jhdcs]

Describe the bug

The function mutable found on https://github.com/eclipse-zenoh/zenoh/blob/45b7ad70bddf45dfcdcae80e9c9a7680c70ac289/commons/zenoh-config/src/lib.rs#L500 transmutes a reference to a mutable reference, which is defined in the Rust language documentation as always being Undefined Behavior (since it breaks LLVM's pointer aliasing rules).

Is it possible to refactor the code to use an UnsafeCell instead?

To reproduce

N/A - found via looking through code

System info

Zenoh Commit: 45b7ad70bddf45dfcdcae80e9c9a7680c70ac289