Added key cache via OS keyring

[applejag]

Changed the repository.Repository implementation to use https://github.com/zalando/go-keyring

This means that password tokens are stored in OS keyring instead of in plain text on your disk.

Screenshot from "KDE Wallet Management Tool", the app used to inspect OS keyring on KDE:

Usage:

- name: oidc
  user:
    exec:
      apiVersion: client.authentication.k8s.io/v1beta1
      args:
        - oidc-login
        - get-token
        - --oidc-issuer-url=https://...............
        - --oidc-client-id=google
        - --oidc-client-secret=...............
        - --force-keyring # <--- new flag
      command: kubectl
      env: null
      interactiveMode: IfAvailable
      provideClusterInfo: false

The code prefers the OS keyring, if supported. Falls back to file based cache. Can be overridden with the new flags:

• --force-keyring
• --no-keyring

Closes #952