Change source packages download path from `src` to `firmware/src` when building for host

When executing:

ros2 run micro_ros_setup create_firmware.sh host

on a colcon workspace, required package's source files, such as eProsima's vendor packages (MicroCDR, Micro-XRCE-DDS-Client) and ros2 packages are downloaded under src.
I would expect the same behaviour as when creating a workspaces for an embedded MCU, which locates everything related with the micro-ROS build under a firmware folder. Thus, deleting this folder assures the user that he/she can start from scratch, while being coherent with the rest of build cases for a micro-ROS environment.

Therefore, I propose moving the location of these files under a firmware/src folder.

Similarly, when creating the workspace for compiling the micro-ros-agent, source artifacts are downloaded under src folder. Having a dedicated uros_agent workspace folder would be desirable.

Inizia con il flusso host di create_firmware.sh invocato da ros2 run micro_ros_setup create_firmware.sh host, quindi traccia la configurazione del workspace utilizzata per compilare micro-ros-agent. Conferma dove vengono attualmente scaricati i pacchetti sorgente e aggiorna il comportamento in modo che i file host utilizzino firmware/src e gli artefatti dell’agente utilizzino un workspace dedicato uros_agent; verifica che l’eliminazione di queste cartelle consenta un riavvio pulito.