Billing Agreements updates

v4.3.7

Add cancel billing agreement function (#222)

Payment source & partner referral

Adding partner referral response (#217)

* Adding links to refund response

* Adding partner referral response

v4.3.5

Updating Order with missing fields (#215)

* Update Order struct with API /v2 response fields according to the documentation https://developer.paypal.com/docs/api/orders/v2/\#orders_get

* Add order completed unmarshal response  unit test

Add links to refund response

v4.3.4

Adding links to refund response (#213)

v4.3.3

Re-adding billing agreements endpoint (#212)

* adding billing_agreements.go file

* added new types

* added tests

* added notice of deprecation

v4.3.2

deletions made (#209)

v4.3.1

Added missing BillingAgreementId key in webhook (#205)

* Added missing BillingAgreementId key in webhook

Webhook resource contains `billing_agreement_id`. Exists in type: `PAYMENT.SALE.COMPLETED` when a subscription is paid.

* review fix

Add support for Create Billing Agreement from Token

Add create billing agreement from token (#204)

* Add create billing agreement from token

v4.2.0

Adding functions for requesting billing agreement token (#203)

* created billing agreements file

* CreateBillingAgreementToken created with new types

* updated formatting

* error to return nil

* removed new line

* added test

v4.1.0

fix typo in PaymentInstruction