Add way to specify executorservice to be used when returning commercetools SDK result

**Is your feature request related to a problem? Please describe.**
Modern async Java applications need control on which Threadpool which tasks are running.
Currently, by default, all responses returning from commercetools SDK will continue running in a thread managed by a threadpool from the SDK.

**Describe the solution you'd like**
Add a way (potentially via Middleware?) to automatically return from any call to commercetools SDK running on a thread from a configurable threadpool AKA Executor

**Describe alternatives you've considered**
Alternatively, we could manually do:

`
ctApi..execute().thenApplyAsync(e -> e, myExecutorService);
`
on each call to commercetools. However, this would clutter the code and can be easily forgotten. Same for any explicit decorator that would be needed.

El issue no menciona archivos, pruebas ni puntos de entrada; comienza rastreando la ruta de resultados asíncronos del SDK y el enfoque de middleware mencionado en la solicitud. Define cómo se proporciona un Executor configurable y verifica que los resultados del SDK se reanuden sistemáticamente en él, cubriendo la alternativa thenApplyAsync existente.

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