feat: add --only-download-deps option to run_demo.sh

[chrisburr]

Summary

Add --only-download-deps option to download helm/kubectl/kind binaries and plugins, then exit early.

This allows CI and other workflows to download dependencies first, then use the downloaded binaries for other operations before running the full demo script.

Changes

• Add --only-download-deps command-line option
• Exit early after downloading dependencies when this option is used
• Update usage documentation

Use Case

This is particularly useful for extensions that need to prepare Helm charts before starting the demo:

# Step 1: Download dependencies
./run_demo.sh --only-download-deps

# Step 2: Use downloaded helm to build chart dependencies
./.demo/helm dependency build /path/to/custom-chart

# Step 3: Run demo with custom chart
./run_demo.sh --chart-path /path/to/custom-chart

Related PRs

• feat: add gubbins-charts Helm chart for gubbins extension diracx#691 - Uses this feature in gubbins CI to build gubbins-charts dependencies