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>
