[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_filedrop.go

@@ -0,0 +1,65 @@
+//go:build ios
+
+package NetBirdSDK
+
+import (
+	log "github.com/sirupsen/logrus"
+
+	"github.com/netbirdio/netbird/client/internal"
+	"github.com/netbirdio/netbird/client/mobile"
+)
+
+// FileDropHandle returns the file drop handle of the client's profile, creating it on first use.
+func (c *Client) FileDropHandle() (*FileDrop, error) {
+	configDir, profileID, err := mobile.ProfileLocationFor(c.cfgFile)
+	if err != nil {
+		return nil, err
+	}
+	return c.fileDropFor(configDir, profileID)
+}
+
+func (c *Client) fileDropFor(configDir, profileID string) (*FileDrop, error) {
+	c.fileDropMu.Lock()
+
+	if c.fileDrop != nil && c.fileDrop.ProfileID() == profileID {
+		fd := c.fileDrop
+		c.fileDropMu.Unlock()
+		return fd, nil
+	}
+
+	fd, err := NewFileDrop(configDir, profileID)
+	if err != nil {
+		c.fileDropMu.Unlock()
+		return nil, err
+	}
+
+	old := c.fileDrop
+	if old != nil {
+		fd.SetListener(old.Listener())
+	}
+	c.fileDrop = fd
+	c.fileDropMu.Unlock()
+
+	if old != nil {
+		if err := old.Close(); err != nil {
+			log.Warnf("failed to close previous file drop manager: %v", err)
+		}
+	}
+
+	return fd, nil
+}
+
+func (c *Client) attachFileDrop(cc *internal.ConnectClient, cfgFile string) {
+	configDir, profileID, err := mobile.ProfileLocationFor(cfgFile)
+	if err != nil {
+		log.Warnf("file drop is unavailable: %v", err)
+		return
+	}
+
+	fd, err := c.fileDropFor(configDir, profileID)
+	if err != nil {
+		log.Warnf("file drop is unavailable: %v", err)
+		return
+	}
+	cc.SetFileDropManager(fd.manager)
+}