Add read-write-backend deployment mode

[pracucci]

We want to introduce a deployment mode which stands in between monolithic and microservices mode. The microservices mode provides the best isolation and fine-grained resources and scaling settings but at the cost of having to deploy 10+ different components. The monolithic mode is at the opposite side, with only 1 component to deploy and scale.

We want to introduce a 3rd deployment mode composed of 3 components:

• Write: distributor + ingester
• Read: query-frontend + querier
• Backend: all other Mimir components

This PR shows a proof-of-concept: #1986

Action plan

• Add read-write-backend deployment mode support (PR)
• Add Alertmanager to backend component (PR)
• Use default HTTP and gRPC ports in the read-write local dev env (PR)
• Add ability to run a fixed number of query-schedulers in the backend component
  • Proposal: add ring-based service discovery for query-scheduler #2950
  • Add ring-based service discovery to query-scheduler #2957
  • Chore: refactor service discovery implementation #2991
  • Add max query-scheduler instances support #3005
• Add basic support in dashboards
  • Basic support (PR)
  • Add "Mimir / Overview" dashboard #3079
• Jsonnet: add multi-zone support

[pracucci]

The read-write deployment mode support has been added to Mimir and it's currently under testing at Grafana Labs. We still have follow up work we want to do, for which I've created dedicated issues, so that it's easier to split the work in the team:

• Improve Mimir dashboards with read-write deployment mode #3361
• Add integration test running Mimir in read-write deployment mode #3363
• Docs: read-write deployment #3362
• Jsonnet: upstream read-write deployment mode support #3364
• Jsonnet: add auto scaling support for read component in the read-write deployment mode #3365
• Ensure all alerts correctly work when Mimir is deployed using the read-write deployment mode #3366