Suggestion: A way to declare dependencies between workflows

## Feature Request

### Problem Overview
In my use case I am declaring workflows via code which uses other workflows as activities.
This means that the WorkflowsProvider has to initiate the dependency first and then the parent, otherwise a `Could not find workflow definition with ID` Exception is thrown.

### Proposed Solution
In my project I have created a solution which uses a DependsOnWorkflowAttribute, which is respected in my own modified implementation of the ClrWorkflowsProvider.

### Alternative Solutions
There are a lot of different solutions to this issue. You could declare dependencies in a property or somehow dynamically check which workflows a workflow uses as activities and build them first.

### Use Cases
I am unsure if this is a general use case or if only I have this issue. I am aware this is probably not a common issue.

### Additional Context
If the Attribute based solution is desirable, I could try and clean up my code a bit and create a PR.