feat: add native WebDAV submodule and usability helpers

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.

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, including robust path traversal and symlink protection.
- A `LockSystem` interface with an in-memory implementation (`MemLock`) that supports resource locking and includes a graceful shutdown mechanism.
- A high-level API in `webdav/easy.go` (`Serve`, `Register`) to simplify serving local directories.
- RFC 4918 compliance for core operations.
- Performance optimizations, including `sync.Pool` for object reuse and `sync/atomic` for lock-free field access.
- Comprehensive unit tests and a working example application.

The Touka framework's core has been updated to recognize all 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.

touka.go

@@ -67,6 +67,8 @@ var (
 	MethodMkcol     = "MKCOL"
 	MethodCopy      = "COPY"
 	MethodMove      = "MOVE"
+	MethodLock      = "LOCK"
+	MethodUnlock    = "UNLOCK"
 )
 
 var MethodsSet = map[string]struct{}{
@@ -84,4 +86,6 @@ var MethodsSet = map[string]struct{}{
 	MethodMkcol:     {},
 	MethodCopy:      {},
 	MethodMove:      {},
+	MethodLock:      {},
+	MethodUnlock:    {},
 }