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. It correctly handles path segments like `.` and `..`.
  - `OSFS`: A secure, OS-based filesystem that interacts with the local disk. It includes robust path traversal protection that correctly handles symbolic links.
- A `LockSystem` interface with an in-memory implementation (`MemLock`) to support resource locking (DAV Class 2). It includes a graceful shutdown mechanism to prevent goroutine leaks.
- RFC 4918 compliance for core operations, including correct status codes for `COPY`/`MOVE` and preventing `DELETE` on non-empty collections.
- Performance optimizations, including the use of `sync.Pool` for object reuse and `sync/atomic` for lock-free field access to reduce GC pressure.
- 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.

This implementation addresses numerous points from detailed code reviews, including security vulnerabilities, memory leaks, RFC compliance issues, and path handling bugs.

webdav/webdav_test.go

@@ -75,7 +75,7 @@ func TestHandlePropfind(t *testing.T) {
 
 	// Create a test directory and a test file
 	fs.Mkdir(nil, "/testdir", 0755)
-	file, _ := fs.OpenFile(nil, "/testdir/testfile", os.O_CREATE|os.O_WRONLY, 0644)
+	file, _ := fs.OpenFile(&touka.Context{Request: &http.Request{}}, "/testdir/testfile", os.O_CREATE|os.O_WRONLY, 0644)
 	file.Write([]byte("test content"))
 	file.Close()