[enhancement]: Call saved workflows via API

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Contact Details

cmsj@tenshu.net

### What should this feature add?

I'd like to be able to define an arbitrarily simple/complex workflow via the Web UI, then call it directly via the API without having to specify the full graph.

I would expect to have to supply a set of required inputs (e.g. at least the prompt matter, but ideally in the same vein as the existing functionality for showing configurable fields in the linear view) as a JSON object and then monitor the execution of the workflow, and taking whatever action I want after it completes (e.g. fetching a saved image, as I would now).

### Alternatives

A simple alternative would be a way to export a workflow as graph json, but that still leads to an unfortunate duplication/desync between workflows saved in Invoke and my codebase.

### Additional Content

_No response_