We need to add support for Per-Object Storage Class (POSC).

Here some extra information to help:

• POSC info from Cloud Storage documentation
• POSC example with JSON API
• Storage Class info from Cloud Storage documentation

On top of that, google-cloud-dotnet has made an implementation which can be used as reference.

• Please view the Pull Request to understand the changes

If you have any implementation questions, please direct them at jskeet

/cc @jskeet @danoscarmike