Plugins that help to pass credentials for S3 and GCS to remote cluster workers

[dbalabka]

I didn't find a simple way to pass credentials to remote workers, such as S3 and GCS, while both are widely used to store data frames.
In this ticket's scope, I propose creating plugins that will help distribute the required keys to remote workers.

GCP credentials
GCP credentials file path is stored in GOOGLE_APPLICATION_CREDENTIALS env variable. The plugin has to create a remote file and pass an env variable with a proper path to workers.

S3 credentials
Like GCP, we must update credential files and store them on each worker.

PR: #439

[jacobtomlinson]

Usually you would create an IAM instance role and profile that can access S3, then configure workers to have this role via the iam_instance_profile keyword argument.

The GCP equivalent is to create a service account that can access GCS and configure that with the service_account kwarg.

This way you don't have to pass credentials around. Is there a reason why you aren't doing it this way?