Create HasWebhookParams interface

[tadhgboyle]

Alongside #3226

Allows events to set specific params to send when being transformed into a JSON object for webhooks.
This is useful for when events contain some params which are objects/classes, since they cannot be nicely formatted into JSON.

For example, if we want to be able to pass the users ID to a webhook when the userRegistered event is fired:

class UserRegisteredEvent implements HasWebhookParams {
  public User $user;

  function __construct(User $user) {
    $this->user = $user;
  }

  function webhookParams(): array {
    return [
      'user_id' => $this->user->data()->id
    ];
  }
}

When previously this would have instead returned something like:

['user' => User object]

[samerton]

To confirm, is this for 2.1.0 or 2.2.0?

[partydragen]

Closes #3275

[tadhgboyle]

To confirm, is this for 2.1.0 or 2.2.0?

2.1.0, as it has no more breaking changes than already existed in the main event revamp PR

outdated