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/meta/main.yml

@@ -141,6 +141,14 @@ argument_specs:
         description: MetalLB ip range for load balancer
         default: 192.168.30.80-192.168.30.90
 
+      metal_lb_interfaces:
+        description: >-
+          List of network interfaces on which MetalLB should announce the
+          load balancer IPs in layer2 mode. When empty (default), MetalLB
+          announces on all interfaces.
+        type: list
+        default: []
+
       metal_lb_controller_tag_version:
         description: Image tag for MetalLB
         default: v0.16.0