Add SKE login command

[Kumm-Kai]

This PR adds a new command (login) below the ske credentials command group.
This command can only be used together with kubectl and a "special" kubeconfig.

To test this PR against prod you can adjust an existing kubeconfig so that it looks like this:

kind: Config
apiVersion: v1
preferences: {}
clusters:
  - name: NAME
    cluster:
      server: URL
      certificate-authority-data: CACERT
      extensions:
        - name: client.authentication.k8s.io/exec
          extension:
            clusterName: NAME
            stackitProjectID: UUID
users:
  - name: NAME
    user:
      exec:
        command: stackit
        args:
          - ske
          - credentials
          - kubeconfig
          - login
        apiVersion: client.authentication.k8s.io/v1
        provideClusterInfo: true
        interactiveMode: Never
contexts:
  - name: NAME
    context:
      cluster: NAME
      user: NAME
current-context: NAME

Additionally, we will soon add a new endpoint that generates a kubeconfig for use with the login command.

CC: @dergeberl

[vicentepinto98]

Currently there are two use cases related to kubeconfig. One is the login for kubectl and the other one is creating a kubeconfig (which we will introduce soon)

When we had the discussion offline to have the command under the credentials command group, I thought we could name the commands something like kubectl-login and then get-kubeconfig. This would be so that we don't introduce another command sub-group.

For reference, the guideline for CLI commands is:
stackit <GROUP> <SUB-GROUP> <COMMAND> <ARGUMENT> <PARAMETER FLAGS> [OPTION FLAGS]

So we should to go with this approach:

stackit ske kubeconfig login and stackit ske kubeconfig create

[Kumm-Kai]

@vicentepinto98 thanks for the initial review. I've implemented the requested changes. 🙂

I'm not 100% happy with the tests, as they create files which won't be cleaned up. The same problem exists also for other tests (e.g., succeeding only locally because the config file already exists).

[vicentepinto98]

Run make generate-docs