Add "Focus" sensor for iOS 15 & macOS 12 (#1664)

Refs #945. This will not be available until we switch to compiling builds with Xcode 13, regardless of when it is merged.

## Summary
Adds a "Focus" binary sensor which is on when focus is enabled. This updates live, even when the app isn't running, via Intents.

## Link to pull request in Documentation repository
Documentation: home-assistant/companion.home-assistant#543

## Any other notes
Followup is needed for onboarding to prompt for this permission.

Sources/Shared/API/Webhook/Sensors/FocusSensor.swift

@@ -0,0 +1,35 @@
+import Foundation
+import PromiseKit
+
+final class FocusSensor: SensorProvider {
+    public enum FocusError: Error, Equatable {
+        case unauthorized
+    }
+
+    let request: SensorProviderRequest
+    init(request: SensorProviderRequest) {
+        self.request = request
+    }
+
+    func sensors() -> Promise<[WebhookSensor]> {
+        guard Current.focusStatus.authorizationStatus() == .authorized else {
+            return .init(error: FocusError.unauthorized)
+        }
+
+        let focusState = Current.focusStatus.status()
+        var sensors = [WebhookSensor]()
+
+        if let isFocused = focusState.isFocused {
+            sensors.append(with(WebhookSensor(
+                name: "Focus",
+                uniqueID: "focus",
+                icon: "mdi:moon-waning-crescent",
+                state: isFocused
+            )) {
+                $0.Type = "binary_sensor"
+            })
+        }
+
+        return .value(sensors)
+    }
+}