Prevent recreating virtual cluster resource during delete

[lizardruss]

What issue type does this pull request address? (keep at least one, remove the others)
/kind bugfix

What does this pull request do? Which issues does it resolve? (use resolves #<issue_number> if possible)
resolves ENG-4924

Please provide a short message that should be published in the vcluster release notes
Fixed an issue where the vcluster syncer would mistakenly re-create a resource when reconciling the update event that adds the deletionTimestamp

What else do we need to know?

Scenario

The syncer incorrectly re-creates a resource while it's being deleted. The sequence of events observed:

• A replicationcontroller with a terminationGracePeriod == 0 is deleted and its pods are garbage collected
• An update event that sets the deletionTimestamp is queued
• An delete event for the same resource is queued
• Syncer reconciles the update event but is unable to load the resource because it's already deleted. This triggers a call to SyncToVirtual(), which mistakenly re-creates the resource.

This scenario occurs more frequently with pods, but the solution has been made generic in case it occurs with different resource kinds.

Solution

Add a PendingDelete event type to communicate that an update event occurred to a virtual resource being deleted. If the SyncToVirtual flow is reached by an event where the source == Virtual and the type == PendingDelete, then skip calling SyncToVirtual.

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