feat: add native WebDAV submodule

This commit introduces a new, high-performance, and extensible WebDAV submodule, implemented natively without external dependencies.

The submodule includes:
- A core WebDAV handler that supports essential methods: PROPFIND, MKCOL, GET, PUT, DELETE, COPY, MOVE, LOCK, and UNLOCK.
- An extensible design using a `FileSystem` interface to decouple the protocol logic from the storage backend.
- Two `FileSystem` implementations:
  - `MemFS`: An in-memory, tree-based filesystem for testing and ephemeral storage.
  - `OSFS`: A secure, OS-based filesystem that interacts with the local disk and includes path traversal protection.
- A `LockSystem` interface with an in-memory implementation (`MemLock`) to support resource locking (DAV Class 2).
- Comprehensive unit tests covering all major functionalities.
- A working example application demonstrating how to mount and use the submodule with a local directory.

The Touka framework's core has been updated to recognize WebDAV-specific HTTP methods.

webdav/memlock.go

@@ -28,9 +28,24 @@ type lock struct {
 
 // NewMemLock creates a new in-memory lock system.
 func NewMemLock() *MemLock {
-	return &MemLock{
+	l := &MemLock{
 		locks: make(map[string]*lock),
 	}
+	go l.cleanup()
+	return l
+}
+
+func (l *MemLock) cleanup() {
+	for {
+		time.Sleep(1 * time.Minute)
+		l.mu.Lock()
+		for token, lock := range l.locks {
+			if time.Now().After(lock.expires) {
+				delete(l.locks, token)
+			}
+		}
+		l.mu.Unlock()
+	}
 }
 
 // Create creates a new lock.
@@ -39,7 +54,9 @@ func (l *MemLock) Create(ctx context.Context, path string, info LockInfo) (strin
 	defer l.mu.Unlock()
 
 	token := make([]byte, 16)
-	rand.Read(token)
+	if _, err := rand.Read(token); err != nil {
+		return "", err
+	}
 	tokenStr := hex.EncodeToString(token)
 
 	l.locks[tokenStr] = &lock{