feat: saveToken plugin for app groups on iOS

This will allow the share extension to access the Bearer token to send
images to the backend.

tauri-plugin-ios-shared-token/ios/Tests/PluginTests/PluginTests.swift

@@ -0,0 +1,8 @@
+import XCTest
+@testable import ExamplePlugin
+
+final class ExamplePluginTests: XCTestCase {
+    func testExample() throws {
+        let plugin = ExamplePlugin()
+    }
+}