Azure Blob storage provider has fixed json extension

[EdoardoTona]

The Azure Blob storage provider (AzureBlobGrainStorage) has fixed .json extension in the blob name:

https://github.com/dotnet/orleans/blob/2d21b3f9afcdd491ded8246ec9ce2401ccbbd388/src/Azure/Orleans.Persistence.AzureStorage/Providers/Storage/AzureBlobStorage.cs#L135

Since the grain storage serializer can be customized (e.g. using the new binary Orleans Serializer), it would be nice to be able to configure the blob file extension from json to something else (including empty string = no extension).