Implement a simple thread pool

# Introduction

In this challenge, you are asked to implement a simple thread pool in C++. Thread pools are a key technique in parallel programming, as they help mitigate the overhead of creating and destroying threads for each task. Your implementation should provide a thread pool that can be initialized with a specified number of worker threads, and an "enqueue" operation that allows tasks to be submitted to the pool. The tasks should be executed by the worker threads, and the results should be returned as a std::future object, allowing the caller to easily retrieve the result.