Update grpc client

[kaibocai]

This PR

1. update the way to build the grpc client so it's easier for dapr workflow to add interceptors (mainly for DAPR_API_TOKEN).
2. decouple orchestrator's name from the orchestrator, so users can use whatever name they like for their orchestrator, also easy the integration with dapr workflow.
3. add missing method documents, resolve API docs #28
4. add human-interaction example, resolve Add human interaction example #23