Add findFirst utility method

**Is your feature request related to a problem? Please describe.**
When you want to find a value just by the key, but don't care about the structure, you most likely write a recursive method to do that for you. So I think it would be good to include this utility method into the project.

**Describe the solution you'd like**
Basically a recursive method for both `AbstractObject` and `AbstractArray`.
`AbstractElement findFirst(String key)`
The method should be on AbstractArray as well, because an AbstractArray could contain objects which contain the key I want to search for.

首先定位 AbstractObject 和 AbstractArray，并阅读它们如何表示嵌套元素。在两个入口点中实现所要求的 findFirst(String key) 行为，递归搜索嵌套对象和数组；完成的标准是两种类型都能无论结构如何找到所请求的键。

由索引模型根据 Issue 内容生成。