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

When executing:
```bash
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.

Comienza con el flujo de host de create_firmware.sh invocado por `ros2 run micro_ros_setup create_firmware.sh host` y, a continuación, sigue la configuración del workspace utilizada para compilar `micro-ros-agent`. Confirma dónde se descargan actualmente los paquetes de código fuente y actualiza el comportamiento para que los archivos del host utilicen `firmware/src` y los artefactos del agente utilicen un workspace dedicado `uros_agent`; verifica que eliminar esas carpetas permita reiniciar limpiamente.

Escrito por el modelo de indexación a partir del texto del issue.