feat(kubectl): support global context and identity flags

[jordi-murgo]

Summary

• add a shared KubectlGlobalArgs parser to kubectl pods, kubectl services, and kubectl logs
• forward --context, --kubeconfig, -n, -l, and --as to underlying kubectl invocations
• add CLI parsing/unit coverage for the new kubectl flag handling

Why

Using rtk kubectl currently drops common kubectl global flags, which breaks context-specific workflows (e.g. --context). This keeps filtered commands aligned with normal kubectl ergonomics.

Validation

• cargo test test_kubectl -- --nocapture
• cargo build --release
• manual checks:
  • rtk kubectl pods --context savage-ovh
  • rtk kubectl services --context savage-ovh