[core] Split DUNE structure from their cfg conterpart

[nrybowski]

Currently, the main structures of DUNE are defined in cfg.rs and are used to directly decode the configuration from a TOML file.
But they are also used to transfer serialized state to remote phynodes with DUNE's internal data that are not present in the configuration file.
Hence, a lot of fields are wrapped into Options to allow TOML configuration decoding in presence of inner data, which implies more useless boilerplate.

Structures representing DUNE elements (and the associated behaviors) must be separated from their configuration conterpart, and should be moved to lib.rs.