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/easy_test.go

@@ -17,6 +17,7 @@ func TestRegister(t *testing.T) {
 	r := touka.New()
 	cfg := &Config{
 		FileSystem: NewMemFS(),
+		LockSystem: NewMemLock(),
 	}
 	Register(r, "/dav", cfg)
 
@@ -35,9 +36,11 @@ func TestServe(t *testing.T) {
 	dir, _ := os.MkdirTemp("", "webdav")
 	defer os.RemoveAll(dir)
 
-	if err := Serve(r, "/serve", dir); err != nil {
+	closer, err := Serve(r, "/serve", dir)
+	if err != nil {
 		t.Fatalf("Serve failed: %v", err)
 	}
+	defer closer.Close()
 
 	// Check if a WebDAV method is registered
 	req, _ := http.NewRequest("OPTIONS", "/serve/", nil)