Make Webhook Event types an enum in the Admin API

[mkurapov]

Context

We should properly define an enum for the different WebhookEvent types (incoming_payment.created, outgoing_payment.completed, etc...) in the GraphQL backend Admin API. This enum would be used in WebhookEvent.type, similar to how IncomingPaymentState is defined for IncomingPayments.

This will provide better clarity in the documentation, and allow consumers of the GraphQL (e.g. MASE) use auto generated TypeScript types for defining the webhooks, instead of having to declare it's own interface.

Todos

• Update the GraphQL Admin API schema
• Update the consumers of the API (MASE, as well as the frontend)

[mkurapov]

Closing, see #3025