feat: add opt-in UNAS Pro support (closes #785)

Adds a new `unas` input plugin that polls UNAS Pro storage consoles and
exports console health, storage pools, disks and shares to Prometheus,
InfluxDB and DataDog.

UNAS is a separate plugin rather than a device type inside inputunifi
because a storage-only console has no Network application: it cannot
answer /status, has no sites, and shares none of the UniFi device schema.

The plugin is opt-in and inert until an operator names a console. Opt-in
is expressed as "no devices configured" rather than a `disable` flag,
because a bool named `disable` zero-values to false and so cannot make a
plugin default-off. Initialize returns silently on an empty device list
and, unlike inputunifi, nothing synthesizes a default URL.

Two behaviours are worth calling out for reviewers:

  - Metrics returns (metrics, nil) whenever any console was collected.
    poller.collectMetrics uses `if err != nil {} else if metric != nil`,
    so returning both would discard every healthy console because one
    failed. Only a total failure returns an error.

  - Re-auth fires on total failure, not on a 401. A mid-session 401 from
    GetData surfaces as ErrInvalidStatusCode, not ErrAuthenticationFailed,
    so there is no sentinel to match on. Session expiry fails all four
    endpoints at once, which is exactly the total-failure case.

Prometheus metrics use the `unifi_unas_` prefix, which diverges from the
`unas_` prefix used by the reference implementation; dashboards built
against that will need query edits.

Requires unifi/v5 v5.31.0 for the UNAS client and structs.

Credit to alexgreenbank/unaspoller for mapping the endpoints.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

examples/up.json.example

@@ -84,5 +84,25 @@
        "verify_ssl":  false
       }
     ]
+  },
+
+  "unas": {
+    "//": "UNAS Pro consoles. JSON cannot carry comments, so this section ships disabled: it shows the shape, and flipping disable to false with a real url turns it on.",
+    "disable": true,
+    "defaults": {
+      "user":       "unpoller",
+      "pass":       "",
+      "verify_ssl": false,
+      "timeout":    "60s"
+    },
+    "devices": [
+      {
+       "url":  "https://192.168.1.10",
+       "user": "unpoller",
+       "pass": "unpoller",
+       "verify_ssl": false,
+       "timeout":    "60s"
+      }
+    ]
   }
 }