refactor: create util module with shared mk_static

- use crate::util::mk_static in network module
- use crate::util::mk_static in mqtt module
- use crate::util::mk_static in hal module
- remove dead mk_static macro from esp module

rust/src/main.rs

@@ -71,6 +71,7 @@ mod mqtt;
 mod network;
 mod plant_state;
 mod tank;
+mod util;
 mod webserver;
 
 extern crate alloc;