[deckhouse-cli] Add -o output format flag to module values

[Glitchy-Sheep]

Summary

• d8 s module values/list/snapshots always printed YAML, with no way to ask for JSON.
• This adds -o/--output (yaml|json) to all three, closing #154.
• The docs can now suggest d8 s module values XXX -o json instead of the long d8 k -n d8-system exec svc/deckhouse-leader -c deckhouse -- deckhouse-controller module values XXX -o json.

How it works

• These commands run curl against the addon-operator debug API inside the deckhouse pod.
• That API picks the response format from the path extension (values.yaml vs values.json).
• So the flag value maps straight into the request path, no local re-serialization.
• New helper utilk8s.GetOutputFormat reads --output and validates it against the accepted formats. It pairs with the existing utilk8s.AddOutputFlag.

Before / After

Before: d8 s module values user-authn only prints YAML.
After: d8 s module values user-authn -o json prints JSON; no flag keeps the old YAML default.

Notes

• text is not offered for module list. The debug route matches .text, but the handler returns a map that the server renders as JSON anyway, so text would promise one format and deliver another.
• Two small fixes came along in the touched files:
  • module list no longer registers the stray --editor flag it never used.
  • module snapshots now requires the module name argument instead of panicking when called without it.

Tests

• TestGetOutputFormat covers the default value, an explicit allowed format, an unsupported format, and a command with no output flag registered.
• Manual run against a live cluster: values, list and snapshots with -o json return JSON; without the flag they return the previous YAML; -o xml is rejected before any cluster call.