cli: introduce exit-node subcommand to list and filter exit nodes

This change introduces a new subcommand, `exit-node`, along with a
subsubcommand of `list` and a `--filter` flag.

Exit nodes without location data will continue to be displayed when
`status` is used. Exit nodes with location data will only be displayed
behind `exit-node list`, and in status if they are the active exit node.

The `filter` flag can be used to filter exit nodes with location data by
country.

Exit nodes with Location.Priority data will have only the highest
priority option for each country and city listed. For countries with
multiple cities, a <Country> <Any> option will be displayed, indicating
the highest priority node within that country.

Updates tailscale/corp#13025

Signed-off-by: Charlotte Brandhorst-Satzkorn <charlotte@tailscale.com>

ipn/ipnlocal/local.go

@@ -747,6 +747,7 @@ func (b *LocalBackend) populatePeerStatusLocked(sb *ipnstate.StatusBuilder) {
 			ShareeNode:   p.Hostinfo.ShareeNode(),
 			ExitNode:     p.StableID != "" && p.StableID == exitNodeID,
 			SSH_HostKeys: p.Hostinfo.SSH_HostKeys().AsSlice(),
+			Location:     p.Hostinfo.Location(),
 		}
 		peerStatusFromNode(ps, p)