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.

从通过 `ros2 run micro_ros_setup create_firmware.sh host` 调用的 create_firmware.sh 主机流程开始，然后跟踪用于编译 `micro-ros-agent` 的工作区设置。确认当前源代码包下载到哪里，并更新行为，使主机文件使用 `firmware/src`，代理构件使用专用的 `uros_agent` 工作区；验证删除这些文件夹后可以进行干净重启。

由索引模型根据 Issue 内容生成。