The only metadata fields that the extension is allowed to write to are the ones that the extension reports as provided. In order to verify that this bargain is upheld, we need to track state of what the extension reports as provided metadata fields.

This ticket should use DocumentSourceExtensionOptimizable to retrieve the logicalStageHandle, call getProvidedMetadataFields() on it, and pass the result through to the ExtensionStage constructor. There, ExtensionStage should save the providedMetadataFields to the object for later use.

This ticket should not implement the verification of metadataWrites in ExtensionStage::doGetNext().