Add support for Service Account authentication with JSON Key File

Service account authentication can be done directly with .p12 files or with 
JSON Key files.

Currently, the .NET library only supports .p12 files.   This feature requests 
is to add support for JSON key files

see
https://cloud.google.com/storage/docs/authentication#generating-a-private-key


Some additional references for JSON key files in java and go libraries:

https://code.google.com/p/google-api-java-client/source/browse/google-api-client
/src/main/java/com/google/api/client/googleapis/auth/oauth2/GoogleCredential.jav
a#776
https://github.com/golang/oauth2/issues/20
https://issues.jenkins-ci.org/browse/JENKINS-24571

cross reference with:  
https://code.google.com/p/google-api-dotnet-client/issues/detail?id=280

Original issue reported on code.google.com by [e-mail] on 23 Feb 2015 at 7:18

[sergey-tihon]

The full source code should be like this

GoogleCredential credential;
using (var stream = new FileStream(fileName, FileMode.Open, FileAccess.Read))
{
    credential = GoogleCredential.FromStream(stream)
        .CreateScoped(VisionService.Scope.CloudPlatform);
}

var service = new VisionService(new BaseClientService.Initializer()
{
    HttpClientInitializer = credential,
    ApplicationName = "my-app-name",
});

[samofwise]

How do you define a user to impersonate? For the Directory Service this is how I used to do it with a .p12 file:

var certificate = new X509Certificate2(HttpContext.Current.Server.MapPath(relativeKeypath), "notasecret", X509KeyStorageFlags.Exportable);

return new ServiceAccountCredential(
new ServiceAccountCredential.Initializer(serviceAccountEmail)
{
    User = user,
    Scopes = scopes
}.FromCertificate(certificate));

If I don’t specify a User I get an Invalid Credentials [401] error. How do I do this now?