feat: report the firmware Netgear offers, not a warning (0.4.0)

The Plus driver looked the newest firmware up in Netgear's download
catalogue and named firmware_update_available itself. It now reports
get_firmware_offer() (napalm-device-types 4.16 FirmwareOfferMixin): what
runs, the newest offered, whether that is newer by Netgear's versioning,
and the download link; netOrk names the warning (netOrk #1111). The Smart
driver's empty get_device_warnings goes too. Requires
napalm-device-types 4.16.0.

tests/unit/test_firmware_offer.py

@@ -0,0 +1,59 @@
+"""The newest firmware Netgear offers, as a fact (netOrk #1111).
+
+The Plus driver looked the newest firmware up in Netgear's download catalogue
+and named ``firmware_update_available`` itself. It reports the offer now --
+what runs, what is offered, whether that is newer by Netgear's versioning --
+and the application decides on the warning.
+"""
+
+from __future__ import annotations
+
+from types import SimpleNamespace
+
+import pytest
+
+from napalm_netgear import netgear_plus
+from napalm_netgear.netgear_plus import NetgearPlusDriver
+from napalm_netgear.netgear_smart import NetgearSmartDriver
+
+
+def _driver(monkeypatch, *, model="GS308E", running="1.00.10", latest="1.00.12"):
+    d = NetgearPlusDriver("sw", "admin", "pw")
+    d._connector = SimpleNamespace(switch_model=SimpleNamespace(MODEL_NAME=model))
+    monkeypatch.setattr(d, "_get_switch_infos", lambda: {"switch_firmware": running})
+    monkeypatch.setattr(netgear_plus, "_fw_get_latest", lambda m: latest)
+    return d
+
+
+def test_a_newer_offer(monkeypatch):
+    assert _driver(monkeypatch).get_firmware_offer() == {
+        "current_version": "1.00.10",
+        "latest_version": "1.00.12",
+        "newer": True,
+        "download_url": f"{netgear_plus._FW_CDN_BASE}/GS308E/GS308E_1.00.12.zip",
+    }
+
+
+def test_running_the_newest(monkeypatch):
+    offer = _driver(monkeypatch, running="1.00.12").get_firmware_offer()
+    assert offer["newer"] is False
+
+
+@pytest.mark.parametrize(
+    "kwargs",
+    [{"latest": None}, {"running": ""}, {"running": "beta"}],
+    ids=["catalogue-unknown", "no-version", "unparsable"],
+)
+def test_cannot_tell(monkeypatch, kwargs):
+    assert _driver(monkeypatch, **kwargs).get_firmware_offer() is None
+
+
+def test_not_connected_cannot_tell(monkeypatch):
+    d = _driver(monkeypatch)
+    d._connector = None
+    assert d.get_firmware_offer() is None
+
+
+@pytest.mark.parametrize("cls", [NetgearPlusDriver, NetgearSmartDriver])
+def test_no_warnings_of_their_own(cls):
+    assert "get_device_warnings" not in vars(cls)