tailcfg: add location field to hostinfo

This change adds Location field to HostInfo.
Location contains the option for a Country, CountryCode, City, CityCode
and a Priority. Neither of these fields are populated by default.

The Priority field is used to determine the priority an exit
node should be given for use, if the field is set. The higher the value
set, the higher priority the node should be given for use.

Updates tailscale/corp#12146

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

tailcfg/tailcfg_test.go

@@ -65,6 +65,7 @@ func TestHostinfoEqual(t *testing.T) {
 		"Cloud",
 		"Userspace",
 		"UserspaceRouter",
+		"Location",
 	}
 	if have := fieldsOf(reflect.TypeOf(Hostinfo{})); !reflect.DeepEqual(have, hiHandles) {
 		t.Errorf("Hostinfo.Equal check might be out of sync\nfields: %q\nhandled: %q\n",