Refactor - Julius Bartolome

[Jules30]

Move most of the business logic to Invoice Entity.

• Added methods to invoice entity to handle Full and Partial payment and remove those logic from ProcessPayment method.
• Created an extension type for enum InvoiceType to detect if the invoice type is supported.
• Added these properties InvoiceHasPayments, InvoiceIsAlreadyFullyPaid and IsPaymentsGreaterThanThePartial to Invoice type to avoid verbose if conditions.