[EventHubs, Azure.Core Shared] Move enqueuedTime from root activity to linked activity (Track Two)

[kinelski]

Fixes #10324

Event Hubs:

• Renamed enqueued time attribute from x-opt-enqueued-time to enqueuedTime.
• Removed the enqueued time attribute from the root processing activity and put it in the linked activity instead.
• Created a new test. It's currently being ignored because it's very similar to RunPartitionProcessingAsyncCreatesScopeForEventProcessing, so it's expected to be flaky during nightly runs. It's already being tracked by [EventHubs] Event Processor tests failing during nightly runs (Track Two) #10067. No issues when running locally.

Core:

• Updated the ClientDiagnosticListener test helper to track linked activities as well, not only their parent ids. I'm keeping the Links property since it's already being used in multiple tests across the repo.

[pakrym]

Change itself LGTM, the only test being ignored concerns me slightly.

[kinelski]

A can enable it for the time being, but I believe it will be flaky during nightly runs for the same (unknown) reason similar tests are. I can put the Ignore attribute back later if that's the case.

[jsquire]

LGTM.

Which test were we thinking would be a problem?

[kinelski]

The new test: RunPartitionProcessingAsyncAddsAttributesToLinkedActivities.

It's basically the same as RunPartitionProcessingAsyncCreatesScopeForEventProcessing, but the assertions at the end are different. That's why I believe it will be flaky.