[EG] ServiceBus required/optional

Next Steps to Merge

Next steps that must be taken to merge this PR:

❌ This is the public specs repo main branch which is not intended for iterative development.
You must acknowledge that you understand that after this PR is merged, you won't be able to stop your changes from being published to Azure customers.
If this is what you intend, add PublishToCustomers label to your PR.
Otherwise, retarget this PR onto a feature branch, i.e. with prefix release- (see aka.ms/azsdk/api-versions#release--branches).
❌ This PR has at least one breaking change (label: BreakingChangeReviewRequired).
To unblock this PR, follow the process at aka.ms/brch.
❌ The required check named TypeSpec Validation has failed. Refer to the check in the PR's 'Checks' tab for details on how to fix it and consult the aka.ms/ci-fix guide

Language	Package Name	ApiView Link
.Net	Azure.Messaging.EventGrid.SystemEvents	https://apiview.dev/Assemblies/Review/3e102a891e5445af8be8cde833dbb702?revisionId=13837d2fe56a4d259b832cd07a77f733
TypeSpec	Azure.Messaging.EventGrid.SystemEvents	https://apiview.dev/Assemblies/Review/bf48b2982ff74cd796f5900a221af2d5?revisionId=eaafc310015544feb68372fa70894f64