protoc-gen-yarpc-go always generates fx code

[Robbert van Ginkel]

After #1449, protoc-gen-yarpc-go always generates code for go.uber.org/fx.

We should make the fx code generation optional to not force a dependency on fx when using yarpc. Either through an opt out like--no-fx or opt in like plugins=fx. Leaning towards the second as its similar as how the default go plugin allows you to also generate grpc code.

[Akshay Shah]

We may need to use an opt-out to preserve backward compatibility.

[bufdev]

In #1449 we talked about adding a --no-fx later, I think this is what you'd want.