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/osfs.go

@@ -9,6 +9,8 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+
+	"github.com/infinite-iroha/touka"
 )
 
 // OSFS is a WebDAV FileSystem that uses the local OS file system.
@@ -98,7 +100,7 @@ func (f *osFile) Readdir(count int) ([]ObjectInfo, error) {
 }
 
 // OpenFile opens a file.
-func (fs *OSFS) OpenFile(ctx context.Context, name string, flag int, perm os.FileMode) (File, error) {
+func (fs *OSFS) OpenFile(c *touka.Context, name string, flag int, perm os.FileMode) (File, error) {
 	path, err := fs.resolve(name)
 	if err != nil {
 		return nil, err