Utilizing Async Instead of Multiprocessing for Subdomain Generation from Multiple Sources

[AmirAflak]

The current implementation of subdomain generation in the "core.py" file of the "subfinder" module uses multiprocessing to speed up the process. However, multiprocessing can be resource-intensive and may not be the most efficient solution for this task. This issue proposes to explore the use of asynchronous programming instead of multiprocessing to improve the performance of subdomain generation from multiple sources. The goal is to reduce resource usage and improve the overall efficiency of the subdomain generation process. This issue will involve refactoring the existing code to use async and testing the performance improvements against the current implementation.