This patch extracts all the DNS related JSON handling from the
cmd/tailscale/jsonoutput package into a new tsdnsjsonv0 package.
It adds package documentation for tsdnsjsonv0 with a big WARNING that
this is an unstable format with no backwards compatibility guarantees.
When we stabilize this format, we should spin off a new tsdnsjsonv1
package that uses jsonoutput.ResponseEnvelope to declare version 1.
Updates #13326
Updates #18750
Signed-off-by: Simon Law <sfllaw@tailscale.com>
f3ec43d7dd