Opamp client api

[LikeTheSalad]

Creation of the OpampClient main API. It's inspired by the existing OpAMP Go client one with the lifecycle methods as well as some setters and a callback, which is also inspired by the callback in the same Go client.

I've also added the 2 proto files from here to be able to generate the proto bindings in this same project as a temporary solution. I'm open to suggestions for better (yet simple) alternatives than having to copy the files over here.