cmd/tailscale/tslockjsonv1: export structs to unmarshal JSON for `tailscale lock`

This patch exports the structs used to unmarshal the JSON output of
the `tailscale lock` subcommand.

Updates #17613

Change-Id: I615723ff9dbc631e452dbf6bbdeab7b97e96dbb3
Signed-off-by: Simon Law <sfllaw@tailscale.com>