feat(kube-vip): add endpoint override for the internal listening address (#699)

* feat(kube-vip): add endpoint override for the internal listening address

Add a kube_vip_endpoint variable so the address kube-vip binds and listens on
can differ from the announced apiserver_endpoint. This is useful for complex
routing and site-to-site tunnels where the VIP kube-vip advertises over ARP
differs from the address it listens on internally.

- roles/k3s_server/templates/vip.yaml.j2: use kube_vip_endpoint (defaulting to
  apiserver_endpoint) for the `address` env and for deriving `vip_subnet`
- roles/k3s_server/defaults/main.yml: add kube_vip_endpoint default (null)
- roles/k3s_server/meta/main.yml: add kube_vip_endpoint argument_spec
- inventory/sample/group_vars/all.yml: document the new sample variable
- README.md: document the kube_vip_endpoint option
- .github/scripts/test-kube-vip-manifest.py: extend regression test to cover the
  default (apiserver_endpoint) and the override case

Closes #221

* chore(ci): extend molecule job timeout to 3 hours

The default scenario occasionally takes longer than 150 minutes on the shared
nested-virt runner (k3s agent notify-wait can exceed the limit under load), and
a single timeout aborts the whole run before the other four scenarios execute.
Raise timeout-minutes from 150 to 180 so a slow-but-progressing run completes
instead of aborting.

The default scenario remains first in the matrix so a failure surfaces fastest.

* fix(kube-vip): fall back on null kube_vip_endpoint and cover it in the test

- vip_subnet and address use default(apiserver_endpoint, true) so the null
  role default falls back to the apiserver endpoint instead of rendering an
  empty/invalid address and subnet
- change the manifest regression test default case to pass kube_vip_endpoint
  as None so it pins the real runtime null condition and fails fast on this
  regression rather than timing out in CI

.github/workflows/test.yml

@@ -88,7 +88,7 @@ jobs:
           trap stop_monitor EXIT
           /usr/bin/time -v -o "$timing_file" \
             molecule test --scenario-name ${{ matrix.scenario }}
-        timeout-minutes: 150
+        timeout-minutes: 180
         env:
           ANSIBLE_K3S_LOG_DIR: ${{ runner.temp }}/logs/k3s-ansible/${{ matrix.scenario }}
           ANSIBLE_SSH_RETRIES: 4