Attempt to make the generated code abstract

[jamezp]

What I mean by this, is that it would be nice if there were some abstract types the generated source files could extend. It makes the hard-coded strings easier to read and validate via a compiler at build time instead of relying on runtime tests.

[ronsigal]

I think that the first class I generated was ServiceGrpcImpl. The reason I generate a new class is that each method in ServiceGrpc has to be overridden. Now, certainly a lot of the code in each method could be factored out into another method, so that each overriding method would be a lot shorter. I haven't worked on that just because it's nice but not crucial.

[jamezp]

That does make sense. I've not looked at the generated code enough to understand if there could be something abstract. I just thought it would be nice if we could do this. It might not be possible, which is also fine.