Add registration method for webhook handlers processing events from the WCPay server

[jrodger]

In #411 we're adding the ability for the plugin to process webhooks events sent from the WCPay server. Related to server#227, it would be good if we didn't make the controller class too cluttered.

We could add a registration mechanism so that all of the actual event processing is handled in dedicated service classes.

[RadoslavGeorgiev]

@jrodger do you think https://github.com/Automattic/woocommerce-payments-server/pull/494 closes this issue? cc @marcinbot

[marcinbot]

Looks like this is similar, but on the client-side. We still process webhooks inside WC_REST_Payments_Webhook_Controller which will probably grow and contain all sorts of business logic that could be easily split across dedicated services for clarity.

That said, we don't really have that many services in the client either. Maybe dedicated handlers would be a good idea, similar to that server change.

[RadoslavGeorgiev]

WC_REST_Payments_Webhook_Controller is now pretty minimal (91 lines incl. comments and white-space), as most of the logic got moved to WC_Payments_Webhook_Processing_Service.

class-wc-payments-webhook-processing-service.php is still on the longer side (650) lines, but most of the code is related to reading out the necessary REST properties, and forwarding them to the right service. At this point it seems like this issue has been addressed.