Support creating DurableOrchestrationClient outside of Azure Functions

[cgillum]

This issue tracks the ability to create an instance of DurableOrchestrationClient outside the Azure Functions context. It looks like we don't have a public constructor for this type, so it's not currently possible.

Here is the relevant source code: https://github.com/Azure/azure-functions-durable-extension/blob/master/src/WebJobs.Extensions.DurableTask/DurableOrchestrationClient.cs#L27.

The challenge will be creating a very simple public constructor that only requires an Azure Storage connection string and a task hub name.

[ConnorMcMahon]

This is getting pushed to October release.

[cgillum]

The PR for this is merged and will go out with v2.4.0.