Add findFirst utility method

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**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.

Comienza localizando AbstractObject y AbstractArray y leyendo cómo representan los elementos anidados. Implementa el comportamiento solicitado de findFirst(String key) en ambos puntos de entrada, con búsqueda recursiva a través de objetos y arrays anidados; se considera terminado cuando cualquiera de los dos tipos puede encontrar la clave solicitada independientemente de la estructura.

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