Allow setting default storage_class for gcs

[Xuanwo]

We can add a new config called default_storage_class for gcs and set it every time users want to insert a new object:

• storage_class: https://cloud.google.com/storage/docs/json_api/v1/objects/insert

[silver-ymz]

I can complete this. Could you assign it me?

[Xuanwo]

Thanks! Have fun!