Google-cloud-node: Allow providing a service account directly for storage object

[AyoubElk]

In short, for a specific use case for a project, I'm handling multiple firebase accounts and the current setup stores the service accounts in a database instead of using the file system.

I need to be able to supply the service account directly to the storage constructor instead of specifying the file name as suggested in the docs:

 var gcs = storage({
   projectId: 'grape-spaceship-123',
   keyFilename: '/path/to/keyfile.json'
 });

[stephenplusplus]

Duplicate of #2536

You can use the options.credentials object when instantiating the client. That is expected to be the contents of a JSON service account key file.