Add eventgrid system topics support

[shanoor]

PR Checklist

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• I have added example(s) inside the [./examples/] folder
• I have added the example(s) to the integration test list for normal (~30 minutes) or long runner >30 minutes
• I have checked the coding conventions as per the wiki
• I have checked to ensure there aren't other open Pull Requests for the same update/change?

Description

Add support for eventgrid system topics and event subscriptions.

Does this introduce a breaking change

• YES
• NO

Testing