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.

touka.go

@@ -60,14 +60,28 @@ var (
 	MethodTrace   = "TRACE"
 )
 
+var (
+	// WebDAV methods
+	MethodPropfind  = "PROPFIND"
+	MethodProppatch = "PROPPATCH"
+	MethodMkcol     = "MKCOL"
+	MethodCopy      = "COPY"
+	MethodMove      = "MOVE"
+)
+
 var MethodsSet = map[string]struct{}{
-	MethodGet:     {},
-	MethodHead:    {},
-	MethodPost:    {},
-	MethodPut:     {},
-	MethodPatch:   {},
-	MethodDelete:  {},
-	MethodConnect: {},
-	MethodOptions: {},
-	MethodTrace:   {},
+	MethodGet:       {},
+	MethodHead:      {},
+	MethodPost:      {},
+	MethodPut:       {},
+	MethodPatch:     {},
+	MethodDelete:    {},
+	MethodConnect:   {},
+	MethodOptions:   {},
+	MethodTrace:     {},
+	MethodPropfind:  {},
+	MethodProppatch: {},
+	MethodMkcol:     {},
+	MethodCopy:      {},
+	MethodMove:      {},
 }