Update Google Cloud object storage plugin

Based on this message, to avoid deprecation, we need to update the build and release process to use the latest C++ client.

We'll start looking into it once we've collected 500 EUR.  It's impossible to evaluate the work without doing it.  We might need up to 1500 EUR to perform the work.  

If we get too many contributions, we'll apply the same update to S3 and Azure plugins as well.  Any extra will be used to maintain Orthanc.