Enhancement/refactoring shipping provider implementation

[ajsuth]

• Refactoring the shipping provider implementation to be agnostic, allowing alternate providers to be managed through configuration.
• Moved the HSShippingAccounts class into the easypost integration temporarily as current solution architecture does not allow common models to be accessed in integration projects.
• Moved CurrencyCode enum into the shared integration library project as is a common enum not specifc to the exchange rates integration project.