Deserializing error for string type

まだ誰も着手していません。

**Describe the bug**
String types in service types cause microROS on the MCU side to fail in the executor due to a deserializing error. Running it through a debugger it fails in ucdr_deserialize_sequence_header when parsing the nested message type.

Service calls with non-string types work as expected. I've verified it's not a buffer overflow since the buffer size >> bytes received. There is also no packet corruption in UDP traffic, the dest packet and src packet match 1:1.

**To Reproduce**
Steps to reproduce the behaviour:
1. On MCU, init service with service type that has a string type
2. On host side, call service

**Expected behaviour**
The MCU accepts the service call and handles accordingly.

**Actual behaviour**
Executor fails due to deserialization error.

**System information (please complete the following information):**
- OS: Ubuntu 22.04
- ROS 2 Humble
- MCU: STM32F767ZI

ROS 2 Humbleを使用し、stringフィールドを含むサービス呼び出しをSTM32F767ZI上で再現してから、ネストされたメッセージ型の解析時にucdr_deserialize_sequence_headerで発生するexecutorの失敗を調査します。MCUがデシリアライズエラーなしでサービス呼び出しを受け付けて処理し、stringを含まないサービス呼び出しが引き続き動作すれば、issueは完了です。

索引モデルが issue の本文から書いたものです。