Refactor Config: extract ConfigInit logic

[Chemaclass]

📚 Description

The current Config class has a lot of responsibilities and it's almost impossible to write isolated tests (unit) for some of its crucial logic. For example, the logic responsible to read different configs by their types.

🔖 Changes

• Apply extract class refactoring pattern, moving out the logic that initializes the config key-values to another class.
• ConfigInit depends on abstractions like PathFinderInterface and GacelaJsonConfigFactoryInterface so it can be easily mock in its unit tests.