[ML] chore: Mark IndexOperations as experimental. (Azure#35439)

* chore: Mark MLClient.indexes as experimental

* chore: Mark IndexOperations functions as experimental

* chore: Remove @experimental on Index asset

* Revert "chore: Remove @experimental on Index asset"

This reverts commit 591be78.

* Revert "chore: Mark IndexOperations functions as experimental"

This reverts commit b25e536.

Per api review feedback that @experimental on methods is probably not
needed if the client property (ml_client.indexes) is marked as
experimental.

Author: kdestin

sdk/ml/azure-ai-ml/azure/ai/ml/_ml_client.py

@@ -1050,6 +1050,7 @@ def marketplace_subscriptions(self) -> MarketplaceSubscriptionOperations:
         return self._marketplace_subscriptions
 
     @property
+    @experimental
     def indexes(self) -> IndexOperations:
         """A collection of index related operations.