[client] Port the file drop bindings to iOS

The iOS SDK package gets the same platform handle the Android bindings
carry: transfers, receiving policy, and per-sender rules on one profile,
attached to the engine when the client starts. Payloads are path-based
rather than stream-based, since the app group container is readable from
both processes, and the platform sink is dropped for the same reason:
delivery lands on the filesystem directly. The default destination is
seeded in the constructor because the app process opens handles of its
own while the extension is down.

client/ios/NetBirdSDK/client.go

@@ -93,6 +93,9 @@ type Client struct {
 	stateMu       sync.RWMutex
 	connectClient *internal.ConnectClient
 	config        *profilemanager.Config
+
+	fileDropMu sync.Mutex
+	fileDrop   *FileDrop
 }
 
 // NewClient instantiate a new Client
@@ -189,6 +192,7 @@ func (c *Client) Run(fd int32, interfaceName string, envList *EnvList) error {
 	connectClient := internal.NewConnectClient(ctx, cfg, c.recorder,
 		internal.WithNetEvents(c.netMgr))
 	c.setState(cfg, connectClient)
+	c.attachFileDrop(connectClient, c.cfgFile)
 	// Persist the latest sync response so DebugBundle can include the network
 	// map. On iOS this is backed by disk to keep it out of the constrained
 	// process memory (see the syncstore package).