Add field to add custom provider and custom model for `mux` module

[LazyGeniusMan]

When using custom provider and/or custom modul in mux module it can be cumbersome to add custom provider and custom model everytime for new workspace. It would be nice if custom provider and custom model can be defined in the module so it's automatically added and ready to be used in the workspace

EDIT: or maybe a post install script hook so I can do setup post installation without manually do it in mux UI (e.g. setting up custom provider, setting up mcp, setting up global mux hook, etc)

[linear-code]

ENG-3030