Export OrchestrationRuntimeStatus for getStatus comparisons

[davidmrdavid]

Resolves: #195

This tiny PR exports the OrchestrationRuntimeStatus enum which allows for safer checks around an orchestration runtime state. This should have been done a long time ago as the runtime_status field of a DurableOrchestrationStatus object was already of OrchestrationRuntimeStatus and yet customers didn't access to the enum declaration itself.

Example:
Checking the status of an orchestration used to be as follows:

# ...
existing_instance = await client.get_status(instance_id)
my_check existing_instance._runtime_status.name == "Pending"
# ...

Whereas now users can do, and are encourage to opt for, the following:

# ...
existing_instance = await client.get_status(instance_id)
my_check existing_instance._runtime_status is df.OrchestrationRuntimeStatus.Pending
# ...

I believe this enum-based comparison is already what happens in JavaScript.

Still ToDo:

• Update docs with an example or reference to this API: https://docs.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-instance-management?tabs=csharp#query-instances

[ConnorMcMahon]

LGTM!