Implement customer endpoints

[devtronic]

Customer endpoints added

GET /api/v1/customers: getCustomers() - Get a list of all customers POST /api/v1/customers: createCustomer() - Creates a new customer GET /api/v1/customers/{id}: getCustomer() - Queries a single customer by id PUT /api/v1/customers/{id}: updateCustomer()- Updates a customer by id GET /api/v1/customers/{id}/orders: getCustomerOrders() - Queries a list of orders from a customer GET /api/v1/customers/{id}/addresses: getCustomerAddresses() - Queries a list of addresses from a customer