Add an action to default the `PaymentMethod`

[nedjulius]

As a customer I need an action to default an existing payment method
So that I know which payment method is going to be used by default for any of my given transactions

Details and Assumptions

• Will need to add a new boolean field is_default
• Need to add test that covers this action and checks if the PaymentMethod is default, and the others are not

• The REST URL to block the payment method will be PUT /payment-method/:payment-method-id/set-default

  Acceptance Criteria

  
  Given an existing payment method that is not a default payment method and there are no other payment methods
  When I send a `PUT /payment-method/12/set-default` request
  Then the payment method field `is_default` should be `True` and I should receive a JSON response that contains the `PaymentMethod` information with it: `{id: 12, name: ... is_default: True}`

Given two existing payment methods, id = 1 and id = 2, where payment method with id = 2 is set to be default When I send a PUT/payment-method/1/set-default request Then the payment method field is_default for id = 1 should be set to True and I should receive a JSON response that contains the PaymentMethod information with it: {id: 1, name: ..., is_default: True}