List all entities from a client

[ahmedmuhsin]

Part of #141

Goal

Let a client query entities with paging and filters.

Scope

• Add a query entities method on the client, sending QueryEntitiesRequest and returning a page of entity metadata plus a continuation token.
• Support the filters the proto query carries, an instance id starts with expression, last modified bounds, include state, include transient, and page size.
• For a standalone backend, implement the QueryEntities RPC over entity storage.

Reference

durabletask-dotnet DurableEntityClient.GetAllEntitiesAsync with EntityQuery, and GrpcDurableEntityClient which pages through QueryEntitiesResponse.

Depends on

The backend entity state storage for the server side.