[ENH] Add support to initialize the workflow context programmatically

In Elsa 2 we have the possibility to set a WorkflowContextId before dispatching a StartableWorkflow.

In Elsa 3 the workflows works as a module and we cant initialize the workflow context programmatically before we dispatch a workflow.

Example of use case:

I have a model Order (my custom context provider) that is the core of my workflow definition so my first activity start iterating all the products of this order to make things.

So it is very important to have the custom context available from the initial moment