Combine setup of all components into a single publically-usable component

[renbou]

Instead of ad-hoc setup of components in main, a semi-generic component in the root package, grpcbridge, should be created, so that it is usable not only from the grpcbridge binary, but also from other packages. It is also a good place where a thorough godoc can be placed regarding how the different components are integrated together in the current setup of proxying to remote services.