feat: licenses, a hypervisor's hosts and the package proxy as facts (4.14.0)

Drivers report facts and the application decides what is a warning
(docs/STRUCTURE.md, NetOrk/netork#1111). Three facts drivers so far
only reported as warning codes get contracts:

- LicenseMixin.get_licenses(): licenses and subscriptions with their
  status (a Proxmox subscription, a VMware edition, a BMC license a
  feature needs).
- HypervisorDriver.get_hypervisor_hosts(): each host connected, in
  maintenance, with configuration issues, under a central manager.
- PackageProxyMixin.get_package_proxy(): the proxy apt fetches through.

All declared only.

tests/test_fact_getters.py

@@ -0,0 +1,40 @@
+"""Facts instead of warnings (docs/STRUCTURE.md, NetOrk/netork#1111).
+
+A driver reports what it reads; the application decides what is a warning. Three
+facts that drivers so far only reported as warning codes get contracts of their
+own: the package manager's proxy, licenses and subscriptions, and the state of a
+hypervisor's hosts. All are declared only, so a capability check stays truthful.
+"""
+
+from __future__ import annotations
+
+from typing import get_type_hints
+
+from napalm_device_types import (
+    HypervisorDriver,
+    HypervisorHostDict,
+    LicenseDict,
+    LicenseMixin,
+    PackageProxyMixin,
+)
+
+
+def test_a_license_says_what_it_is_and_whether_it_holds():
+    hints = get_type_hints(LicenseDict)
+    assert {"name", "status"} <= set(hints)
+    assert LicenseDict.__required_keys__ == frozenset({"name", "status"})
+    assert set(hints) == {"name", "status", "edition", "expires", "detail"}
+
+
+def test_a_hypervisor_host_says_how_it_stands():
+    hints = get_type_hints(HypervisorHostDict)
+    assert set(hints) == {"name", "connected", "maintenance", "config_issues", "managed_by"}
+    assert HypervisorHostDict.__required_keys__ == frozenset(
+        {"name", "connected", "maintenance", "config_issues"}
+    )
+
+
+def test_the_getters_are_declared_only():
+    assert not hasattr(PackageProxyMixin, "get_package_proxy")
+    assert not hasattr(LicenseMixin, "get_licenses")
+    assert not hasattr(HypervisorDriver, "get_hypervisor_hosts")