case insensitive field matching behavior different from iceberg-python and iceberg-java

https://github.com/apache/iceberg-cpp/pull/180 added support for `GetFieldByName` with `case_sensitive` argument. Its behavior is different from Python and Java Iceberg implementations which could result in inconsistent behavior when used together.

Python/Java are unicode/multibyte aware. iceberg-cpp is not. An example where this breaks e.g. column names containing e.g. "Latin Capital Letter I with Dot Above" https://www.compart.com/en/unicode/U+0130.

Does not affect me directly at this moment.

从 GetFieldByName 的实现以及 apache/iceberg-cpp#180 引入的行为开始。以包含 U+0130 的列名为例，将不区分大小写的匹配与 Python 和 Java Iceberg 实现进行比较。完成的标准是 iceberg-cpp 能够以与这些实现一致的方式处理 Unicode 和多字节名称，并且为该示例提供覆盖。

由索引模型根据 Issue 内容生成。