Arduino Wire.h I2C custom transport on Pi Pico is causing a crash

- Hardware description: Pi Pico
- RTOS: I think arduino mbed
- Version or commit hash: humble

#### Steps to reproduce the issue
I'm trying to make a custom transport for I2C using arduino's Wire library. I am able to build and flash the microros_publisher example, everything works over serial and I can see the topic and node in a docker container. However, when I replace setting the serial transport with my own, I think the pico crashes since i'm getting a flashing pattern on the pico (4 long, 4 0r 5 long). I really hope I'm missing something subtle. Also I'm not sure how I am supposed to properly use `uxr_init_custom_transport`, or if I'm supposed to use it at all, because the default serial transport doesn't. Also if this isn't the correct place to post this issue, please point me in the right direction.

here's my ini

```
env:pico]
platform = https://github.com/maxgerhardt/platform-raspberrypi.git
board = pico
framework = arduino
lib_deps =
https://github.com/micro-ROS/micro_ros_platformio

upload_protocol = mbed

board_microros_distro = humble
board_microros_transport = custom
```

heres the code

https://pastebin.com/5hzPiRcD

从 PlatformIO ini 和链接的 Pastebin 代码开始，重点查看自定义传输的配置方式，以及 `uxr_init_custom_transport` 的使用位置。先复现串行基线，然后在 Pi Pico 上实现 I2C 传输；当自定义传输不再导致开发板崩溃，并且 micro-ROS 节点和 topic 仍然可见时，即表示完成。

由索引模型根据 Issue 内容生成。