feat(config system): Add thread pool config system

• Add configuration system to thread pool.
• Add new ctor to allow initializing thread pool from a JSON file
• Validate config system
• Add config JSON file in ./config
• Add performance benchmark to thread pool test
• Refactor code and move all related structs, classes and functions into namespace "threadpool"
• Add documentation for thread pool functions and test classes
• Unify comment style