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/osfs.go

@@ -26,8 +26,16 @@ func NewOSFS(rootDir string) (*OSFS, error) {
 }
 
 func (fs *OSFS) resolve(name string) (string, error) {
+	if filepath.IsAbs(name) {
+		return "", os.ErrPermission
+	}
 	path := filepath.Join(fs.RootDir, name)
-	if !strings.HasPrefix(path, fs.RootDir) {
+
+	rel, err := filepath.Rel(fs.RootDir, path)
+	if err != nil {
+		return "", err
+	}
+	if strings.HasPrefix(rel, "..") {
 		return "", os.ErrPermission
 	}
 	return path, nil