[WIP] First version of a protocol for allow external UI(s) for the visualize the stepper process

[alejandrozf]

This PR implements some generic hooks for the contrib abcl-stepper that can be used to build a protocol of communication between the stepper process and an external UI.

There is a reference implementation using HTMX and websockets of this idea in https://gitlab.com/cl-projects/abcl-visual-stepper which only needs to load these changes to abcl-stepper