feat(kubectl): add global context and identity flags (#857)

[pszymkowiak]

Summary

Adds KubectlGlobalArgs struct with global kubectl flags shared across all subcommands:

Flag	Description
--context	kubeconfig context (multi-cluster)
--kubeconfig	custom kubeconfig path
-n / --namespace	namespace scope
-l / --selector	label selector
--as	user impersonation

These flags now work on rtk kubectl pods, rtk kubectl services, and rtk kubectl logs.

Example: rtk kubectl pods --context prod -n api -l app=web

Based on PR #426 by @jordi-murgo (inactive since March 9), rebased on develop.
Closes #857 (partial — global flags done, TOML filters for describe/apply/top still needed).

Test plan

• 1126 tests passed, 0 failed
• cargo fmt clean
• cargo clippy clean
• 3 new tests: flag parsing, to_args conversion, all-namespaces + context combo

[aeppling]

Hey

We are cleaning up the codebase and improving the project structure for better onboarding. As part of this effort, PR #826 reorganizes src/ from a flat layout into subfolders.

No logic changes — only file moves and import path updates.

What you need to do

Rebase your branch on develop when receiving this comment:

git fetch origin && git rebase origin/develop

Git detects renames automatically. If you get import conflicts, update the paths:

use crate::git;        // now: use crate::cmds::git::git;
use crate::tracking;   // now: use crate::core::tracking;
use crate::config;     // now: use crate::core::config;
use crate::init;       // now: use crate::hooks::init;
use crate::gain;       // now: use crate::analytics::gain;

Need help rebasing? Tag @aeppling