feat(metallb): add option to limit MetalLB layer2 announcements to specific interfaces (#696)

- roles/k3s_server_post/templates/metallb.crs.j2: render spec.interfaces in the
  L2Advertisement when metal_lb_interfaces is a non-empty list, so MetalLB only
  announces on the configured interfaces. Empty list (default) keeps announcing
  on all interfaces, preserving existing behavior.
- roles/k3s_server_post/defaults/main.yml: add metal_lb_interfaces: [] default
- roles/k3s_server_post/meta/main.yml: add metal_lb_interfaces argument_spec
- inventory/sample/group_vars/all.yml: document the new sample variable
- .github/scripts/test-metallb-interfaces.py: regression test rendering the
  template for empty/single/multiple interfaces and confirming the BGP path is
  unaffected
- .pre-commit-config.yaml: wire the new test into pre-commit

Co-authored-by: Leo <leo@kuboschek.me>

roles/k3s_server_post/templates/metallb.crs.j2

@@ -21,6 +21,11 @@ kind: L2Advertisement
 metadata:
   name: default
   namespace: metallb-system
+{% if metal_lb_interfaces | default([]) | length > 0 %}
+spec:
+  interfaces:{% for iface in metal_lb_interfaces %}
+    - {{ iface }}{% endfor %}
+{% endif %}
 {% endif %}
 {% if metal_lb_mode == "bgp" %}
 ---