Create a go client for creating service

[NicolasMahe]

A new dedicated go client should be create to ease the creation of MESG Services using golang, in a similar to than https://github.com/mesg-foundation/mesg-js.

A simple client package is already used by the system service ethwallet:
https://github.com/mesg-foundation/engine/tree/v0.14.1/systemservices/ethwallet

On the the big issue of this client is it's using map[string]interface instead of pbStruct.
An attempt to fix this problem has been done but discarded but of the time require to manually transform the pbStruct to geth struct: mesg-foundation/engine#1324

Solutions:

• automatically convert one struct to the other. something like https://godoc.org/github.com/jkawamoto/structpbconv
• create a bunch of helpers to convert (like the previous PR) or the lib https://godoc.org/github.com/robteix/protoconv