Feature request: runtime switching of transport channel

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## Support for runtime switching of transport (UDP / custom, etc )

Current implementation requires compile time definition for transport type. Adding support for runtime
switching will open possibility of applications to setup backup channel in case main channel fails or create more dynamic applications.

### possible implementation
- add a another transport "multimode" apart from UDP, custom, etc which defines multimode implementation. Thus only users with "multimode" bear the overhead of binary size and execution.

#### multimode
- [rmw_uxrce_transport_params_t](https://github.com/micro-ROS/rmw_microxrcedds/blob/jazzy/rmw_microxrcedds_c/include/rmw_microros/rmw_microros.h#L55) contains parameters for all possible types. Nothing excluded by macros.

- Add a static global variable "RMW_UXRCE_ACTIVE_TRANSPORT" to determine currently active transport and make transport call as per currently active transport.

+ many more

Does this sound like a good idea ?

Inizia esaminando rmw_microros.h, in particolare rmw_uxrce_transport_params_t, e traccia il funzionamento attuale della selezione a tempo di compilazione del trasporto UDP e del trasporto personalizzato. Valuta il trasporto multimode proposto e l’approccio RMW_UXRCE_ACTIVE_TRANSPORT; per considerare concluso il lavoro sarebbe necessario concordare un design per il cambio a runtime e il comportamento del canale di backup.

Scritto dal modello di indicizzazione a partire dal testo della issue.