feat(cilium): add toggle to enable or disable the Envoy proxy (#695)

Add a `cilium_envoy` variable (default true, matching upstream Cilium 1.20
which installs Envoy by default) that controls whether the Envoy proxy is
deployed for Cilium L7 policies. Pass it to Helm as `envoy.enabled` so users
with no L7 policies can skip Envoy to save resources.

- roles/k3s_server_post/defaults/main.yml: add cilium_envoy: true default
- roles/k3s_server_post/tasks/cilium.yml: add --helm-set envoy.enabled to the
  install/upgrade command, driven by the cilium_envoy conditional
- inventory/sample/group_vars/all.yml: document cilium_envoy sample var
- .github/scripts/test-cilium-envoy-toggle.py: regression test asserting the
  install command carries the envoy.enabled helm-set and renders true/false
- .pre-commit-config.yaml: wire the new test into pre-commit

Co-authored-by: Léo Nonnenmacher <leo@nonnenmacher-logel.fr>

inventory/sample/group_vars/all.yml

@@ -24,6 +24,10 @@ cilium_mode: native # native when nodes are on the same subnet or use BGP, other
 cilium_tag: v1.20.0 # cilium version tag
 cilium_cli_tag: v0.19.7 # cilium cli version tag
 cilium_hubble: true # enable hubble observability relay and ui
+cilium_envoy: true # enable the Envoy proxy for Cilium L7 policies
+
+# disable cilium_envoy to skip the Envoy proxy entirely (e.g. no L7 policies)
+# cilium_envoy: false
 
 # if using calico or cilium, you may specify the cluster pod cidr pool
 cluster_cidr: 10.52.0.0/16