all: Support syncing ownership (fixes #1329) (#8434)

This adds support for syncing ownership on Unixes and on Windows. The
scanner always picks up ownership information, but it is not applied
unless the new folder option "Sync Ownership" is set.

Ownership data is stored in a new FileInfo field called "platform data". This
is intended to hold further platform-specific data in the future
(specifically, extended attributes), which is why the whole design is a
bit overkill for just ownership.

lib/fs/basicfs_platformdata_unix.go

@@ -0,0 +1,18 @@
+// Copyright (C) 2022 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+//go:build !windows
+// +build !windows
+
+package fs
+
+import (
+	"github.com/syncthing/syncthing/lib/protocol"
+)
+
+func (f *BasicFilesystem) PlatformData(name string) (protocol.PlatformData, error) {
+	return unixPlatformData(f, name)
+}