* structs: CSIPlugin indexes jobs acting as plugins and node updates

* schema: csi_plugins table for CSIPlugin

* nomad: csi_endpoint use vol.Denormalize, plugin requests

* nomad: csi_volume_endpoint: rename to csi_endpoint

* agent: add CSI plugin endpoints

* state_store_test: use generated ids to avoid t.Parallel conflicts

* contributing: add note about registering new RPC structs

* command: agent http register plugin lists

* api: CSI plugin queries, ControllerHealthy -> ControllersHealthy

* state_store: copy on write for volumes and plugins

* structs: copy on write for volumes and plugins

* state_store: CSIVolumeByID returns an unhealthy volume, denormalize

* nomad: csi_endpoint use CSIVolumeDenormalizePlugins

* structs: remove struct errors for missing objects

* nomad: csi_endpoint return nil for missing objects, not errors

* api: return meta from Register to avoid EOF error

* state_store: CSIVolumeDenormalize keep allocs in their own maps

* state_store: CSIVolumeDeregister error on missing volume

* state_store: CSIVolumeRegister set indexes

* nomad: csi_endpoint use CSIVolumeDenormalizePlugins tests