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.

Start with the GetFieldByName implementation and the behavior introduced by apache/iceberg-cpp#180. Compare case-insensitive matching with the Python and Java Iceberg implementations, using a column name containing U+0130 as the example. Done means iceberg-cpp handles Unicode and multibyte names consistently with those implementations, with coverage for the example.

Written by the indexing model from the issue text.