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.
  - `OSFS`: A secure, OS-based filesystem that interacts with the local disk and includes path traversal protection.
- A `LockSystem` interface with an in-memory implementation (`MemLock`) to support resource locking (DAV Class 2).
- 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.

examples/webdav/main.go

@@ -0,0 +1,37 @@
+package main
+
+import (
+	"log"
+	"os"
+	"time"
+
+	"github.com/infinite-iroha/touka"
+	"github.com/infinite-iroha/touka/webdav"
+)
+
+func main() {
+	r := touka.Default()
+
+	// Create a directory for the OS file system.
+	if err := os.MkdirAll("public", 0755); err != nil {
+		log.Fatal(err)
+	}
+
+	// Create a new WebDAV handler with the OS file system.
+	fs, err := webdav.NewOSFS("public")
+	if err != nil {
+		log.Fatal(err)
+	}
+	handler := webdav.NewHandler("/webdav", fs, webdav.NewMemLock(), log.New(os.Stdout, "", 0))
+
+	// Mount the WebDAV handler on the "/webdav/" route.
+	webdavMethods := []string{
+		"OPTIONS", "GET", "HEAD", "DELETE", "PUT", "MKCOL", "COPY", "MOVE", "PROPFIND", "PROPPATCH", "LOCK", "UNLOCK",
+	}
+	r.HandleFunc(webdavMethods, "/webdav/*path", handler.ServeTouka)
+
+	log.Println("Touka WebDAV Server starting on :8080...")
+	if err := r.RunShutdown(":8080", 10*time.Second); err != nil {
+		log.Fatalf("Touka server failed to start: %v", err)
+	}
+}