Signal an entity from a client

[ahmedmuhsin]

Part of #141

Goal

Let a client signal an entity operation without going through an orchestration.

Scope

• Add a signal entity method to the client, sending the SignalEntityRequest RPC with instance id, operation name, input, an optional scheduled time, and a request id.
• For a standalone durabletask-go backend, implement the SignalEntity RPC on grpcExecutor and enqueue the operation.

Reference

durabletask-dotnet DurableEntityClient.SignalEntityAsync and GrpcDurableEntityClient, which fills SignalEntityRequest.

Depends on

The backend entity scheduler for the server side. The client side needs the RPC and the foundational entity runtime.