考虑允许拓展`ReflectLoader#loadField`?

从解析到执行链看是 `Parser -> GetFieldInstruction -> ReflectLoader#loadField` 里进行任何对象属性取值，目前看实现是封闭的；

对于一些`容器 Like`，但非标准容器结构体，如：
- MapLike、CollecttionLike
- FlinkRow、SparkRow、Structed
- JDBC ResultSet

如果想允许实现`structObj.col_a.nested_col_b`的取值，并不好进行直访拓展，需要额外进行一侧 convert copy，没有那么方便；

Trace the Parser -> GetFieldInstruction -> ReflectLoader#loadField execution path first, focusing on how object properties are currently read. Review the issue's examples—MapLike, FlinkRow, SparkRow, Structed, and JDBC ResultSet—and define an extension point that supports nested property access without requiring conversion copies. Done means the proposed behavior and extension boundary are clear.

Written by the indexing model from the issue text.