Provide native service builders for gRPC clients #121
Description
PR #120 introduced gRPC support for diagram-based workflow builders. That feature does not help native Rust API users.
We should provide a generic service builder function that allows user to easily spawn services that implement the same functionality that the diagram workflow builder has. However we should consider whether we can make the input and output types generic so users can select a service definition at runtime and we can infer the input and output types, as well as whether it's unary or streaming.