Add kubeconfig and context overrides to the CLI

[Copilot]

kubectl-ditto always used the default Kubernetes client config, which made it impossible to target a specific kubeconfig file or context on machines with multiple clusters. This change adds explicit CLI overrides and routes client initialization through kube’s kubeconfig-aware config APIs when requested.

• CLI surface

  • Adds --kubeconfig <PATH> to select a specific kubeconfig file
  • Adds --context <NAME> to select a specific kubeconfig context
  • Keeps existing invocation shape unchanged for users relying on default config resolution

• Client configuration

  • Preserves kube::Client::try_default() when no override flags are provided
  • Uses Config::from_custom_kubeconfig(...) when --kubeconfig is set
  • Uses Config::from_kubeconfig(...) with KubeConfigOptions { context, .. } when only --context is set
  • Supports combining both flags so context selection applies to an explicit kubeconfig file

• Docs and coverage

  • Updates README usage to show kubeconfig/context targeting
  • Adds focused CLI parsing tests for the new flags

kubectl ditto deployment my-app \
  --kubeconfig ~/.kube/prod-config \
  --context production