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.

[dbalabka]

moved to dask/dask-cloudprovider#438