Add Durable Entities support

[ahmedmuhsin]

Summary

durabletask-go has no Durable Entities support today. The task registry takes only orchestrators and activities, the work item dispatch loop drops entity work items as an unknown type, and the backend, gRPC executor, and clients have no entity methods. The shared protobuf contract already carries every entity message the other SDKs use, so this is unimplemented surface in this SDK rather than a protocol gap.

This tracks adding entities end to end. It uses Microsoft.DurableTask (durabletask-dotnet) as the reference since it already ships the full feature on the same gRPC contract.

Motivation

• Support for Durable Entities? #64 asked for entity parity with the .NET version and is still open.
• Downstream workers need it. The Azure Functions Go worker sequences entity support as its last durable phase and is blocked on entity primitives landing here first. Dapr Workflow is in the same position.

What exists today

• Protos already define SignalEntity, GetEntity, QueryEntities, CleanEntityStorage, EntityBatchRequest, EntityBatchResult, WorkItem.EntityRequest, and the entity history and lock events.
• Nothing consumes them. TaskRegistry has AddOrchestrator and AddActivity only, StartWorkItemListener handles orchestrator and activity requests and logs everything else as an unknown work item, and grpcExecutor returns Unimplemented for the entity RPCs.

Work breakdown

Entities land as one foundational sub issue plus one sub issue per capability in the feature matrix. The foundation carries the entity runtime that the rest build on. The capability sub issues stay focused on the user facing surface. The sub issues are tracked on this item.

Reference

Microsoft.DurableTask implements all of the above on the same protobuf contract. The types to mirror are DurableTaskRegistry.AddEntity, ITaskEntity and TaskEntity<TState>, TaskEntityContext, TaskEntityOperation, TaskOrchestrationEntityFeature for the orchestration side, and DurableEntityClient for the client side.

Sub issues

• Foundational entity runtime #142 Foundational entity runtime
• Call an entity from an orchestration #143 Call an entity from an orchestration
• Signal an entity from an orchestration #144 Signal an entity from an orchestration
• Signal an entity from a client #145 Signal an entity from a client
• Get entity state from a client #146 Get entity state from a client
• List all entities from a client #147 List all entities from a client
• Entity locking (critical sections) #148 Entity locking (critical sections)
• Class based entities #149 Class based entities
• Entity storage cleanup #150 Entity storage cleanup