Decouple Ruler from ConfigDB & add separate ruler API and storage backend

[jtlisi]

This PR accomplishes the following

• Add a separate API for the Ruler and Alertmanager based on this design document
• Add a GCS backend for the ruler API
• Update the polling mechanism to use a KV store backed change detection system for the decouple APIs or use a generation based configdb client
• Update the scheduler to use a user-based context to allow for cancellations after updates or deletions
• Pass a prometheus register with the users ID to the prometheus rule group to get user level prom eval metrics

Any feedback or suggestions are greatly appreciated