This commit adds `--json` output mode to dns debug commands.
It defines structs for the data that is returned from:
`tailscale dns status` and `tailscale dns query <DOMAIN>` and
populates that as it runs the diagnostics.
When all the information is collected, it is serialised to JSON
or string built into an output and returned to the user.
The structs are defined and exported to golang consumers of this command
can use them for unmarshalling.
Updates #13326
Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>
d82e478dbc