Cross cluster rule evaluation

[jmoscoso1]

Is your feature request related to a problem? Please describe.
Currently, Cortex rulers are limited to their own Cortex cluster. They can only evaluate rules and alert from the Ingesters of their own cell. This causes limited flexibility in how Cortex clusters can be run.

Describe the solution you'd like
In order to allow Cortex Rulers to communicate with other cortex clusters, we should allow custom implementations of the Appender and Querier. These can be passed in to Cortex through a newly introduced config. Using these custom implementations, the Ruler can remote-write and remote-read to other external endpoints. This would allow for cortex clusters to used solely for rule evaluation and alerting and vice versa others for just ingestion and querying. (see picture).

These custom implementations of Appender and Querier are not necessary and default implementations will still be able to be used if necessary.