Add ListEntities API

As in title, the following API is missing from the durable client: https://docs.microsoft.com/en-us/dotnet/api/microsoft.azure.webjobs.extensions.durabletask.idurableentityclient.listentitiesasync?view=azure-dotnet

**As part of that work, we should also make sure that the API accessible to HTTP clients.**

Start by comparing the durable client's available APIs with the referenced .NET ListEntitiesAsync documentation, then trace how durable-client APIs are exposed to HTTP clients. Done means the missing ListEntities API is available through the durable client and is also accessible to HTTP clients.

Written by the indexing model from the issue text.