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.

Comece localizando AbstractObject e AbstractArray e lendo como eles representam elementos aninhados. Implemente o comportamento solicitado de findFirst(String key) nos dois pontos de entrada, com busca recursiva por objetos e arrays aninhados; a tarefa estará concluída quando qualquer um dos dois tipos puder encontrar a chave solicitada independentemente da estrutura.

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