Changes to Create a checkout session

the customerRef request property's maxLength was set to 20

the planRef request property's maxLength was set to 20

the productRef request property's maxLength was set to 20

the endpoint scheme security SecretKey was added to the API

the endpoint scheme security bearer was removed from the API

the customerRef request property's minLength was increased from 0 to 1

the productRef request property's minLength was increased from 0 to 1

added the new optional request property purpose

the request property productRef became optional

the response property checkoutUrl became required for the status 201

the response property sessionId became required for the status 201

endpoint added