A credit card object represents a credit card that is saved to a customer's account.
Properties:
id - Integer - Unique id for the credit card
last4 - Integer - Last 4 digits of the card card number
first_name - String - First name of the cardholder
last_name - String - Last name of the cardholder
month - Integer - Month of the card expiration date
year - Integer - Year of the card expiration date
brand - String - Brand of credit card
display_number - String - Helper method to display masked credit card number
expiration - String - Helper method to display card expiration date
name_on_card - String - Helper method to combine first and last name