ipn/localapi: add support for multipart POST to file-put

This allows sending multiple files via Taildrop in one request.
Progress is tracked via ipn.Notify.

Updates tailscale/corp#18202

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

ipn/ipnlocal/local.go

@@ -319,6 +319,9 @@ type LocalBackend struct {
 	// lastNotifiedTailFSShares keeps track of the last set of shares that we
 	// notified about.
 	lastNotifiedTailFSShares atomic.Pointer[views.SliceView[*tailfs.Share, tailfs.ShareView]]
+
+	// outgoingFiles keeps track of Taildrop outgoing files
+	outgoingFiles map[string]*ipn.OutgoingFile
 }
 
 type updateStatus struct {