Fixes to Get/Delete for Guid keys

[adamsitnik]

This PR fixes a bug reported offline by @jeffhandley

fail: Microsoft.Extensions.DataIngestion.IngestionPipeline[6]
      An error occurred while ingesting document 'Example_GPS_Watch.md'.
      System.InvalidCastException: Unable to cast object of type 'System.Guid' to type 'System.String'.
         at System.Linq.Enumerable.CastICollectionIterator`1.MoveNext()
         at Azure.Search.Documents.Models.IndexDocumentsBatch.Delete(String keyName, IEnumerable`1 keyValues)
         at Azure.Search.Documents.SearchClient.DeleteDocumentsAsync(String keyName, IEnumerable`1 keyValues, IndexDocumentsOptions options, CancellationToken cancellationToken)
         at Microsoft.Extensions.VectorData.VectorStoreErrorHandler.RunOperationAsync[TResult,TException](VectorStoreCollectionMetadata metadata, String operationName, Func`1 operation)
         at Microsoft.Extensions.DataIngestion.VectorStoreWriter`1.WriteAsync(IAsyncEnumerable`1 chunks, CancellationToken cancellationToken)
         at Microsoft.Extensions.DataIngestion.IngestionPipeline`1.IngestAsync(IngestionDocument document, Activity parentActivity, CancellationToken cancellationToken)
         at Microsoft.Extensions.DataIngestion.IngestionPipeline`1.ProcessAsync(IEnumerable`1 files, Activity rootActivity, CancellationToken cancellationToken)+MoveNext()