Add a "tailscale whoami" subcommand that is equivalent to running "tailscale whois $(tailscale ip -4)" but more ergonomic. It supports the --json flag just like whois, and shares the WhoIsResponse rendering code with whois. Fixes #19907 Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com> Change-Id: I8f33ba7a5608bab7dffa8213303beb5f345936d3