Simplify registration of IWorkflowResumer

## Enhancement Request

### Enhancement Overview
We have a need to inject a custom implementation of IWorkflowResumer. Currently this is not super straight forward. I believe that it would be useful to allow registration of a custom workflow resumer by setting a property in `WorkflowRuntimeFeature`.

### Proposed Enhancement
You should be able to configure the workflow resumer implementation like this:
```
elsa.UseWorkflowRuntime(workflowRuntime =>
{
workflowRuntime.WorkflowResumer = sp => sp.GetRequiredService();
});
```