Use config consistently through plugins

Plugin callbacks has inconsistent naming:

   fn on_cfg_validate(&self, configuration: Self::CFG) -> CallbackResult<()>;
   fn on_config_change(
       &mut self,
       ctx: &PicoContext,
       new_cfg: Self::CFG,
       old_cfg: Self::CFG,
   ) -> CallbackResult<()>;

I hereby declare that we shall use config instead of cfg everywhere to avoid inconsistency

DoD:

• Change RFC
• Change code so it uses only config

RFC discussion: https://docs.google.com/document/d/1QMFXWhrSJ9VLFNX-jayHCzG8jk01VwtOuTOzooBBsOA/edit?disco=AAABLZaHC3w