add `delete_all_documents()` operation to `AzureAISearchDocumentStore`

- seems like one way is to simply delete the index, and recreate it again, might have other side effects
- https://learn.microsoft.com/en-us/python/api/azure-search-documents/azure.search.documents.indexes.searchindexclient?view=azure-python#azure-search-documents-indexes-searchindexclient-delete-index