Internal: Refactor how webhooks work

[turt2live]

This PR is a step towards replacing the "audit context". The approximate plan is to move to a "record/publish" pattern instead of an audit pattern. Recorders can then remember some details about harms on events. Webhooks would be one of the "publishers", so not much effort is made to keep the audit code clean. Implementing recorders/publishers is for a future PR.

This PR adds an interface for notifiers with an assumption that eventually policyserv will be able to send events itself, which would be a new notifier type.

Pull Request Checklist

• Pull request is based on the main branch.
• Pull request title describes the changes in a way a user would understand. For example, "Fix keywords filter not applying" instead of "Read keywords slice in a loop".
• Code style matches surrounding code.
• Tests are added for new code (and existing code) if possible.
  • If not possible, please explain why in your PR description.
• The CI checks pass.