cmd/tailscale: add --json for up

Print JSON to stdout containing everything needed for
authentication.

{
  "AuthURL": "https://login.tailscale.com/a/0123456789",
  "QR": "data:image/png;base64,iV...QmCC",
  "BackendState": "NeedsLogin"
}
{
  "BackendState": "Running"
}

Signed-off-by: Denton Gentry <dgentry@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f