feat: add Swift Package Manager support - Phase 2

- Add Package.swift with iOS 14+ target and proper resource handling
- Configure SPM target for workmanager_apple with Sources/workmanager_apple path
- Include Resources/PrivacyInfo.xcprivacy processing for SPM
- Support Swift tools version 5.9

✅ Verified: SPM builds successfully with flutter config --enable-swift-package-manager
✅ Verified: CocoaPods backward compatibility maintained
✅ Verified: Both dependency managers work independently

Users can now choose their preferred dependency manager:
- CocoaPods: flutter config --no-enable-swift-package-manager
- SPM: flutter config --enable-swift-package-manager

This completes the core SPM migration while maintaining full backward compatibility.

Author: ened

workmanager_apple/ios/Package.swift

@@ -0,0 +1,26 @@
+// swift-tools-version: 5.9
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+    name: "workmanager_apple",
+    platforms: [
+        .iOS(.v14)
+    ],
+    products: [
+        .library(
+            name: "workmanager_apple",
+            targets: ["workmanager_apple"]
+        )
+    ],
+    targets: [
+        .target(
+            name: "workmanager_apple",
+            path: "Sources/workmanager_apple",
+            resources: [
+                .process("../Resources")
+            ]
+        )
+    ]
+)