Core mode should allow persistent usage of custom K8s client configuration

# Summary

The Argo CD core mode should allow the user to set a permanent path to a K8s client configuration file, instead of either using `~/.kube/config` per default or having to specify `--kubeconfig` for each command.

# Motivation

Some K8s distributions by default install a Kubernetes client CLI (`kubectl`) which uses a custom path for its configuration. For example, K3s uses `/etc/rancher/k3s/k3s.yaml`. Every change to a given context therefore modifies this file, instead of `~/.kube/config`, which can become confusing in core mode.

# Proposal

Argo CD CLI should store the default path to a Kubernetes client config in its own config (i.e. `~/.argocd/config`) with the `login` command, so that other commands in core mode will work out-of-the-box with custom paths, e.g.:

```
argocd login --core --kubeconfig /etc/rancher/k3s/k3s.yaml
```

should store the path to the kubeconfig file for that Argo CD CLI context. If it's set in the context, all Argo CD CLI commands in core mode should configure the Kubernetes client from that file, instead of `~/.kube/config`, unless `--kubeconfig` is overriding this path explicitly.

In detail:

* The `argocd login` command should be extended for a `--kubeconfig` flag to take the path of a Kubernetes client configuration file
* If given, the path should be saved with the context in the Argo CD CLI's configuration file. If not set, the existing default (i.e. `~/.kube/config) should be used.
* If the path to a K8s client configuration is set in the context of Argo CD CLI, it should be used for all commands as default in core mode by default
* If `--kubeconfig` flag is given to Argo CD CLI commands, it should override the default stored in the Argo CD CLI's configuration