Give support to directories in taildrop direct file mode
If a directory has been delivered in direct file mode, the archive should be pre-extracted in `*taildrop.Manager.PutFile`, as no "tailscale file get" command will be executed manually.
Incidentally, clients that use direct file mode to receive files (e.g. Android Client) will automatically get directory support in taildrop.
Signed-off-by: yqs112358 <37969157+yqs112358@users.noreply.github.com>
