upload artifacts using new blob artifact kind

[jonasfj]

See if we can't use: https://github.com/taskcluster/taskcluster-lib-artifact-go/

We should just add a method: TaskContext.UploadBlobArtifact as it'll be used in many different plugins.

[jonasfj]

And we should implement the blob type in the fake queue implementation, so that integration tests will continue to pass without TC credentials.