Add per-definition toggle for webhook notifications

Description

Context

Currently, the webhook notifications are being sent for every approval from the instance.

However, in practice customers might need to receive webhooks only from some particular approvals.

Scope

Add an additional toggle on definition level to control whether approvals started with the definition should send webhook notifications.

  • The default value of the switch should be true to ensure backwards compatibility.

  • The setting is evaluated at approval start - switching it mid-approval does not change the webhook functionality.