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>

.pre-commit-config.yaml

@@ -93,6 +93,14 @@ repos:
         language: system
         pass_filenames: false
         files: ^roles/k3s_server/tasks/metallb\.yml$|^\.github/scripts/test-metallb-remote-read\.sh$
+      - id: metallb-interfaces-test
+        name: MetalLB interfaces test
+        entry: python3 .github/scripts/test-metallb-interfaces.py
+        language: python
+        additional_dependencies:
+          - Jinja2>=3.1
+        pass_filenames: false
+        files: ^roles/k3s_server_post/templates/metallb\.crs\.j2$|^\.github/scripts/test-metallb-interfaces\.py$
       - id: metallb-deploy-condition-test
         name: MetalLB deploy condition test
         entry: python3 .github/scripts/test-metallb-deploy-condition.py