tailfs: initial implementation

Add a WebDAV-based folder sharing mechanism that is exposed to local clients at
100.100.100.100:8080 and to remote peers via a new peerapi endpoint at
/v0/tailfs.

Add the ability to manage folder sharing via the new 'share' CLI sub-command.

Updates tailscale/corp#16827

Signed-off-by: Percy Wegmann <percy@tailscale.com>

tailfs/tailfs.go

@@ -0,0 +1,18 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package tailfs provides a filesystem that allows sharing folders between
+// Tailscale nodes using WebDAV.
+package tailfs
+
+import (
+	"time"
+)
+
+const (
+	// statCacheTTL causes the local WebDAV proxy to cache file metadata to
+	// avoid excessive network roundtrips. This is similar to the
+	// DirectoryCacheLifetime setting of Windows' built-in SMB client,
+	// see https://learn.microsoft.com/en-us/previous-versions/windows/it-pro/windows-7/ff686200(v=ws.10)
+	statCacheTTL = 10 * time.Second
+)