ApiWorkflowProvider

We currently have 3 workflow providers:

**ProgrammaticWorkflowProvider**: provides workflow blueprints based on the workflows coded with the fluent Workflow Builder API.
**DatabaseWorkflowProvider**: provides blueprints based on those stored in the database (JSON models stored by the designer).
**StorageWorkflowProvider**: provides blueprints based on JSON files stored on some hard drive or blob storage such as Azure Blob Storage.

We should have a fourth provider that uses the Elsa API endpoints to get workflows from.

Then client apps that run Elsa locally won't have to invoke the Elsa API, but instead take advantage of the workflows provider model.