ctlv3: support "write-out" for "endpoint health" command

[gyuho]

$ ETCDCTL_API=3 ./bin/etcdctl endpoint --cluster health --write-out simple                                                    
http://127.0.0.1:2379 is healthy: successfully committed proposal: took = 474.51µs                                                                            
http://127.0.0.1:32379 is healthy: successfully committed proposal: took = 719.75µs                                                                           
http://127.0.0.1:22379 is healthy: successfully committed proposal: took = 459.323µs                                                                          

$ ETCDCTL_API=3 ./bin/etcdctl endpoint --cluster health --write-out table                                                     
+------------------------+--------+-----------+-------+                        
|        ENDPOINT        | HEALTH |   TOOK    | ERROR |                        
+------------------------+--------+-----------+-------+                        
|  http://127.0.0.1:2379 |   true | 520.835µs |       |                        
| http://127.0.0.1:32379 |   true | 518.894µs |       |                        
| http://127.0.0.1:22379 |   true | 751.764µs |       |                        
+------------------------+--------+-----------+-------+                        

$ ETCDCTL_API=3 ./bin/etcdctl endpoint --cluster health --write-out proto                                                     
Error: unsupported output format       

$ ETCDCTL_API=3 ./bin/etcdctl endpoint --cluster health --write-out json                                                      
[{"endpoint":"http://127.0.0.1:32379","health":true,"took":"749.662µs"},{"endpoint":"http://127.0.0.1:2379","health":true,"took":"456.438µs"},{"endpoint":"http://127.0.0.1:22379","health":true,"took":"772.233µs"}]

Fix #9532.

/cc @omkensey