Add `tailscale version --json` JSON output mode. This will be used
later for a double-opt-in (per node consent like Tailscale SSH +
control config) to let admins do remote upgrades via `tailscale
update` via a c2n call, which would then need to verify the
cmd/tailscale found on disk for running tailscale update corresponds
to the running tailscaled, refusing if anything looks amiss.
Plus JSON output modes are just nice to have, rather than parsing
unstable/fragile/obscure text formats.
Updates #6995
Updates #6907
Change-Id: I7821ab7fbea4612f4b9b7bdc1be1ad1095aca71b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Brad Fitzpatrick