Add request/response type to method definition

[kurayama]

Adds the resolved request/response type to grpc-loader method definition.

[thelinuxfoundation]

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[nicolasnoble]

@kurayama thank you for your contribution, but in order for us to accept it, we'd need you to follow the instructions provided by the CLA bot ☝️. Thanks!

[kurayama]

@nicolasnoble done

[murgatroid99]

Thank you for your contribution, but we cannot accept it. One of the design goals of this library was to avoid directly exposing the API of Protobuf.js, and this change would break that. I understand the value of exposing this information, and I would be open to suggestions about other ways to do it.