Add filter on webhook pending (843)

[mmaymo]

This PR adds a filter with the order.

[Biont]

issue:

_order_status_pending does not tell a clear story what is this is about.
It might lead developers into thinking we are referring to order status here when it is in fact about payment status.

onWebhookPending() also lacks any documentation about this distinction.

We should add comprehensive comments about the nature of both entities and their interpretations of the pending state - and how both interact. This is to underline and defend our decision to not do anything - while allowing custom processing if needed.

issue:

I see we are exposing a way for developers to change the order being processed - and we later add our note to it. I understood this new API is to allow third-party developers to add custom handling for pending payments. But I am struggling to see the use case to allow switching orders here. This feels like do_action to me more than apply_filters