feat: add ability for users to specify alternative kubeconfig and kubecontext

This will provide the possibility for users to talk to existing kubernetes clusters, thereby removing one of the biggest reasons for having the old scheduler-kubernetes plugin around.

docs/deployment/schedulers/k3s.md

@@ -320,6 +320,38 @@ By default, Dokku assumes that all it controls all actions on the cluster, and t
 dokku scheduler-k3s:show-kubeconfig
 ```
 
+### Interacting with an external Kubernetes cluster
+
+While the k3s scheduler plugin is designed to work with a Dokku-managed k3s cluster, Dokku can be configured to interact with any Kubernetes cluster by setting the global `kubeconfig-path` to a path to a custom kubeconfig on the Dokku server. This property is only available at a global level.
+
+```shell
+dokku scheduler-k3s:set --global kubeconfig-path /path/to/custom/kubeconfig
+```
+
+To set the default value, omit the value from the `scheduler-k3s:set` call:
+
+```shell
+dokku scheduler-k3s:set --global kubeconfig-path
+```
+
+The default value for the `kubeconfig-path` is the k3s kubeconfig located at `/etc/rancher/k3s/k3s.yaml`.
+
+### Customizing the Kubernetes context
+
+When interacting with a custom Kubeconfig, the `kube-context` property can be set to specify a specific context within the kubeconfig to use. This property is available only at the global leve.
+
+```shell
+dokku scheduler-k3s:set --global kube-context lollipop
+```
+
+To set the default value, omit the value from the `scheduler-k3s:set` call:
+
+```shell
+dokku scheduler-k3s:set --global kube-context
+```
+
+The default value for the `kube-context` is an empty string, and will result in Dokku using the current context within the kubeconfig.
+
 ## Scheduler Interface
 
 The following sections describe implemented and unimplemented scheduler functionality for the `k3s` scheduler.