Paulvancoller techtest submission

[ghost]

This PR submits Paul van Coller's (Cyberpro Consulting) Tech Test results.

It refactors the code into some better practices and implements Customer listing (with optional orders), as well as customer linking to orders.

Refactoring updates:

• Object reorganization for clarity.
• Hardcoded settings moved to settings file (connection strings and VAT settings).
• Parameterized all queries for sanitation.
• Class comments to simplify intention.

Issues / refactoring not done:

Description	Reason
Inversion of Control	Due to this being a class library, dependency injection would not be set up here, but rather on the parent project utilizing this library. One might want to use interfaces on sub-objects to allow these dependencies to be handed to the OrderService, but the static class on the CustomerRepository does not allow for this. Additionally, since the Order Repository is marked as internal, one might guess that the original intention was for only the AnyCompany assembly to know about these objects, rendering Inversion of Control on these objects irrelevant.
Unit tests	Unit tests are difficult to create here due to the lack of Inversion of Control, as discussed above. One would preferably mock any connection or repository objects in order to test only the specific unit of code.
Integration tests	Despite the absence of unit tests, one could still implement integration tests, either via Postman (using a web API parent project and an on-the-spot database instance), or CLI-Unit (using a CLI parent project and an on-the-spot database instance).
Country Code Enum	A choice was made not to change the internal country code properties to an Enum, but rather leave it as a string. This is due to the fact that the data will be passed from an external project, and that multiple databases are used.
Key Guids	Since customers and orders live in different databases, it would be recommended that their primary keys be changed from ints to GUIDs, in order to ensure uniqueness of the keys across multiple databases or servers.

[paulvancoller]

@seanogt, please review when you get a chance?