Support Paypal webhooks for APMs

[yoannisj]

Description

Although not required, the Paypal Checkout integration guide recommends implementing webhooks to consolidate the integration and as a workaround for Alternative Payment Methods (i.e. "APM") edge cases:

While the ability to display and accept APMs is included with the checkout integration, PayPal recommends subscribing to the "CHECKOUT.ORDER.APPROVED" webhook event in case a customer accidentally closes the browser and exits the checkout process after approving the transaction through their APM but before finalizing the transaction on your site.

Currently I found disabling the APMs with the javascript sdk disable-funding query argument is the easiest way to make sure this edge case is avoided, but it's a bit of a pity to give up on support for APMs, and it is not really maintainable in the long run as it requires a white-list of APM handles (the list needs to be updated if Paypal starts supporting a new APM).

[luis33338]

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