Handle webhook inputs consistently and in one place

[Pijukatel]

Unify Webhook input sanitation and handling. Maybe with Pydantic?

Context:
Webhook inputs to the clients are now handled in different ways based on the client used. One path is using encode_webhook_list_to_base64, for example in ActorClient.start.

The other one is using get_webhook_representation, for example in WebhookClient.update.

This creates situation where same responsibility is handled in two different places and it can happen than one of the paths has issues that are already fixed in the other one. For example: #419