Support preserving API endpoints when upgrading cluster

[deliahu]

Description

Currently, when upgrading to a newer version of Cortex, it is recommended to spin up a new cluster. Once API gateway is used in front of APIs, it may be possible to preserve the API and operator endpoints during a cluster upgrade. It may also be possible to do a "rolling" cluster upgrade, so that traffic continues to be served. This would require a new CLI command, e.g. cortex cluster upgrade.