feat: add native WebDAV submodule with usability helpers and fixes

This commit introduces a new, high-performance, and extensible WebDAV submodule, implemented natively without external dependencies. It also adds a high-level API to simplify common use cases and incorporates numerous fixes based on detailed code reviews.

Features:
- A core WebDAV handler supporting `PROPFIND`, `MKCOL`, `GET`, `PUT`, `DELETE`, `COPY`, `MOVE`, `LOCK`, `UNLOCK`.
- An extensible design with `FileSystem` and `LockSystem` interfaces.
- `MemFS`: A robust, tree-based in-memory filesystem for testing.
- `OSFS`: A secure OS-based filesystem with protection against path traversal and symlink attacks.
- `MemLock`: An in-memory locking system with graceful shutdown to prevent resource leaks.
- A high-level API (`webdav.Serve`, `webdav.Register`) for ease of use.

Fixes & Improvements:
- Security: Patched directory traversal and symlink vulnerabilities. Ensured secure lock token generation.
- RFC Compliance: Corrected status codes for `COPY`/`MOVE` (201 vs 204), `DELETE` on non-empty collections (409), and `Timeout` header parsing.
- Performance: Implemented `sync.Pool` for object reuse and `sync/atomic` for file size management to reduce GC pressure.
- Robustness: Fixed numerous bugs related to path handling, resource cleanup (goroutine leaks), and header parsing.

Integration:
- The Touka framework's core has been updated to recognize all necessary WebDAV methods.
- Includes comprehensive unit tests and a working example.

webdav/webdav.go

@@ -588,11 +588,8 @@ func (h *Handler) handlePropfind(c *touka.Context) {
 }
 
 
-func (h *Handler) createPropfindResponse(path string, info ObjectInfo, propfind Propfind) *Response {
-	fullPath := path
-	if h.Prefix != "/" {
-		fullPath = h.Prefix + path
-	}
+func (h *Handler) createPropfindResponse(p string, info ObjectInfo, propfind Propfind) *Response {
+	fullPath := path.Join(h.Prefix, p)
 
 	resp := &Response{
 		Href:      []string{fullPath},
@@ -641,10 +638,7 @@ func (h *Handler) handleProppatch(c *touka.Context) {
 }
 
 func (h *Handler) stripPrefix(p string) string {
-	if h.Prefix == "/" {
-		return p
-	}
-	return strings.TrimPrefix(p, h.Prefix)
+	return strings.TrimPrefix(strings.TrimPrefix(p, h.Prefix), "/")
 }
 
 func (h *Handler) handleLock(c *touka.Context) {