Support describing nested messages

[yebrahim]

Using desc is only useful on top level messages. Also support calling it for nested objects:

myservice@localhost:50051> desc SayHelloRequest
+---------------------+----------------------------+----------------+
|        FIELD        |              TYPE          | REPEATED       |
+---------------------+----------------------------+----------------+
| person              | TYPE_MESSAGE               | false          |
|                     | (Person)                   |                |
+---------------------+----------------------------+----------------+

Should be able to do something like desc Person, or desc com.HelloService.Person.

[ktr0731]

Currently, there are no ways to do it, but I think the behavior should be improved.

FYI: in the command-line mode, we can describe all messages such as:

$ evans -r cli desc
api.Example:
service Example {
  rpc SayHello ( .api.SayHelloRequest ) returns ( .api.SayHelloResponse );
}

$ evans -r cli desc api.SayHelloRequest
api.SayHelloRequest:
message SayHelloRequest {
  Person person = 1;
}

$ evans -r cli desc api.Person
api.Person:
message Person {
  // something
}

[yebrahim]

yeah it'd be real nice if we can do it in the repl, especialy with those long-nested message types. :)