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.

Start with the create_firmware.sh host flow invoked by `ros2 run micro_ros_setup create_firmware.sh host`, then trace the workspace setup used for compiling the `micro-ros-agent`. Confirm where source packages are currently downloaded and update the behavior so host files use `firmware/src` and agent artifacts use a dedicated `uros_agent` workspace; verify that deleting those folders permits a clean restart.

Written by the indexing model from the issue text.