feat(helm): add --helm-dependency-update to run helm dependency update

[isumitsolanki]

Description

Adds opt-in --helm-dependency-update so Kustomize runs helm dependency update on each chart before helm template, and exposes the same behavior via HelmConfig.DependencyUpdate for API users.

Fixes #6119

Problem

kustomize build --enable-helm renders charts with helm template but does not refresh chart dependencies. Local charts (or setups like Argo CD that only run kustomize build --enable-helm) fail when Chart.yaml lists dependencies but the charts/ directory was never populated by helm dependency update.

Root cause

The Helm inflator assumes the chart directory is already dependency-complete. helm dependency update is a separate, potentially expensive step (disk + network). Kustomize intentionally did not run it by default.

Solution

• New CLI flag: --helm-dependency-update (requires --enable-helm or --enable-alpha-plugins).
• Before template: run helm dependency update <chartDir> after the chart exists locally (including after helm pull).
• New field: types.HelmConfig.DependencyUpdate.
• Test: local chart with file:// dependency — fails without flag, succeeds with DependencyUpdate.

How to test

go test ./kustomize/commands/build/... ./api/krusty/... -run TestHelmDependencyUpdate -count=1

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