D12: Attribute-based plugin discovery + config pre-existing names fix

Andrii Podanenkorequested to merge
d12-attribute-plugin-discovery into 4.x
  • findPreExistingConfiguration() honors $previous_config_names (CR#3492559)
  • ConfigEventIgnorePluginManager: annotation -> Attribute (combined with earlier commits on this branch)

D12 readiness sweep - opening the PR that was missing for this already-pushed branch.

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