Proposal: introduce `INameVersionProvider`

### Background
I am working on enabling open-generic orchestrations and activities in DTFx. This will allow for registering an open generic to the TaskHubWorker (ie: `MyTaskActivity`) and being able to schedule closed generic versions of it (`context.ScheduleTask(typeof(MyTaskActivity), "some input");`). I have it working for the most part, just the overloads where DTFx takes in the type and uses `NameVersionHelper.GetDefaultName(object obj)` does not work - as this uses `Type.ToString()` when I need `Type.FullName`. The difference between the two is `FullName` will include assembly info in the generic arguments, which is essential for loading that type in the worker.

### Proposal
1. Introduce a new interface:

``` CSharp
public interface INameVersionProvider
{
string GetName(object obj, bool useFullyQualifiedMethodNames = false);

string GetVersion(object obj);
}
```

2. Plum that interface through all places where `NameVersionHelper` is used.
- `TaskHubClient` and `TaskHubWorker` should accept this interface - either a ctor overload or a public property.
- The provider supplied to the `TaskHubWorker` should flow in to the `OrchestrationContext` and also be used there.
- Supplying this will be optional.
3. When not supplied, `NameVersionHelper` will be used instead.

The goal will be to introduce no break changes and make this interface entirely optional - so users will only see a different behavior if they opt to supply a custom `INameVersionProvider`.

I have a PR nearly ready for this - will put it up soon.