*: support getting role in auth v3

This commit implements RoleGet() RPC of etcdserver and adds a new
subcommand "role get" to etcdctl v3. It will list up permissions that
are granted to a given role.

$ ETCDCTL_API=3 bin/etcdctl role get r1
Role r1
KV Read:
        b
        d
KV Write:
        a
        c
        d

etcdserver/etcdserverpb/rpc.proto

@@ -603,6 +603,7 @@ message AuthRoleAddRequest {
 }
 
 message AuthRoleGetRequest {
+  string role = 1;
 }
 
 message AuthRoleDeleteRequest {
@@ -664,6 +665,8 @@ message AuthRoleAddResponse {
 
 message AuthRoleGetResponse {
   ResponseHeader header = 1;
+
+  repeated authpb.Permission perm = 2;
 }
 
 message AuthRoleDeleteResponse {