feat: add DomainEvent activity

[cfgiugliano]

This adds the DomainEvent activity as proposed in #5583

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This change is 

[cfgiugliano]

@dotnet-policy-service agree

[sfmskywalker]

Would it make sense to use INotificationSender instead of IWebhookDispatcher?

[cfgiugliano]

This was added to provide feature parity with the RunTask and I thought this was just a default notification handler, but what would be the reason for this additional indirection of a notification publishing another notification?

[sfmskywalker]

I was more thinking that we want to be able to publish domain events, which are "lower-level" than webhook events. Ergo, it doesn't make sense to me to use a service called "webhook dispatcher" when it has nothing to do with webhooks. For that reason, I'm thinking that using INoticationSender is more suitable. The webhook dispatcher uses this too under the hood. Alternatively, we could consider adding a new service called e.g. IDomainEventDispatcher (which would probably still use INotificationSender, just like IWebhookDispatcher).

[cfgiugliano]

But the activity execution is already publishing a the domain event as notification using INotificationSender (this could be chanced to consume a IDomainEventDispatcher, yes) but the current handler is just a default implementation of the notification handler like for RunTask. If required, it is still possible to implement a custom handler and handle the domain event with custom logic