Runtime: Support persistent storage for distributed workers

[begelundmuller]

As we look toward separating ETL and serving, and running ETL on worker nodes, the runtime will need the ability to persist ETL state in GCS.

Requirements:

• Use a system-wide GCS connection for ETL state for all instances (to make configuration easier and to prevent corruption).
• Ensure complete isolation of data between instances
• Ability to track/limit usage per instance
• On local, ability to use a sub-directory of the tmp directory for ETL state instead of GCS

Tasks:

• Design proposal (one idea is to leverage SystemConnectors and configure it to a local file driver on local and GCS on cloud)