[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/filedrop_payload.go

@@ -0,0 +1,71 @@
+//go:build ios
+
+package NetBirdSDK
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"os"
+
+	"github.com/netbirdio/netbird/client/internal/filedrop"
+)
+
+// FileDropPayloads collects the items of one outgoing transfer.
+type FileDropPayloads struct {
+	items []filedrop.Payload
+}
+
+// NewFileDropPayloads returns an empty payload list to fill before sending.
+func NewFileDropPayloads() *FileDropPayloads {
+	return &FileDropPayloads{}
+}
+
+// AddFile appends a file item backed by a filesystem path the extension can read.
+func (p *FileDropPayloads) AddFile(name string, size int64, contentType, path string) error {
+	if name == "" {
+		return errors.New("file name is required")
+	}
+	if path == "" {
+		return fmt.Errorf("file %s has no path", name)
+	}
+
+	p.items = append(p.items, filedrop.Payload{
+		Meta: filedrop.FileMeta{
+			Name:        name,
+			Size:        size,
+			ContentType: contentType,
+		},
+		Open: func(offset int64) (io.ReadCloser, error) {
+			f, err := os.Open(path)
+			if err != nil {
+				return nil, err
+			}
+			if offset > 0 {
+				if _, err := f.Seek(offset, io.SeekStart); err != nil {
+					_ = f.Close()
+					return nil, err
+				}
+			}
+			return f, nil
+		},
+	})
+	return nil
+}
+
+// AddText appends an inline text item.
+func (p *FileDropPayloads) AddText(name, text string) error {
+	if len(text) > filedrop.MaxInlineTextSize {
+		return fmt.Errorf("text exceeds %d bytes", filedrop.MaxInlineTextSize)
+	}
+	if name == "" {
+		name = "text"
+	}
+	p.items = append(p.items, filedrop.TextPayload(name, text))
+	return nil
+}
+
+// Length returns the number of items.
+func (p *FileDropPayloads) Length() int {
+	return len(p.items)
+}