cli: add status cmd

The new command allows checking the status of an upgrade
and which versions are installed.
Also remove the unused restclient.
And make GetConstellationVersion a function.

docs/docs/reference/cli.md

@@ -34,6 +34,7 @@ Commands:
     * [azure](#constellation-iam-create-azure): Create IAM configuration on Microsoft Azure for your Constellation cluster
     * [gcp](#constellation-iam-create-gcp): Create IAM configuration on GCP for your Constellation cluster
   * [destroy](#constellation-iam-destroy): Destroy an IAM configuration and delete local Terraform files
+* [status](#constellation-status): show status of a Constellation cluster
 
 ## constellation config
 
@@ -673,3 +674,31 @@ constellation iam destroy [flags]
       --force           disable version compatibility checks - might result in corrupted clusters
 ```
 
+## constellation status
+
+show status of a Constellation cluster
+
+### Synopsis
+
+Show status of a constellation cluster.
+
+Shows microservice, image and Kubernetes versions installed in the cluster. Also show status of current version upgrades.
+
+```
+constellation status [flags]
+```
+
+### Options
+
+```
+  -h, --help   help for status
+```
+
+### Options inherited from parent commands
+
+```
+      --config string   path to the configuration file (default "constellation-conf.yaml")
+      --debug           enable debug logging
+      --force           disable version compatibility checks - might result in corrupted clusters
+```
+

docs/docs/workflows/upgrade.md

@@ -57,3 +57,38 @@ If you are interested, you can monitor pods restarting in the `kube-system` name
 Image and Kubernetes upgrades take longer.
 For each node in your cluster, a new node has to be created and joined.
 The process usually takes up to ten minutes per node.
+
+## Check the status
+
+Upgrades are asynchronous operations.
+After you run `upgrade apply`, it will take a while until the upgrade has completed.
+To understand if an upgrade is finished, you can run:
+
+```bash
+constellation status
+```
+
+This command displays the following information:
+
+* The installed services and their versions
+* The image and Kubernetes version the cluster is expecting on each node
+* How many nodes are up to date
+
+Here's an example output:
+
+```shell-session
+Target versions:
+	Image: v2.6.0
+	Kubernetes: v1.25.8
+Installed service versions:
+	Cilium: v1.12.1
+	cert-manager: v1.10.0
+	constellation-operators: v2.6.0
+	constellation-services: v2.6.0
+Cluster status: Some node versions are out of date
+	Image: 23/25
+	Kubernetes: 25/25
+```
+
+This output indicates that the cluster is running Kubernetes version `1.25.8`, and all nodes have the appropriate binaries installed.
+23 out of 25 nodes have already upgraded to the targeted image version of `2.6.0`, while two are still in progress.