[FEATURE] Allow Open-WebUI as an optional UI

[kpcrash]

Feature Description

As a user and administrator of systems, I want to provide my users with the Open-WebUI features and functionality while maintaining the Gradio interface for select users. This could be added as an extra parameter in the poetry install.

Reason

There are many inherent features in Open-WebUI that don't need to be reinvented in PrivateGPT

Value of Feature

PGPT would have built-in RBAC, Workspaces for smaller GPT projects, and a mobile interface with relatively low effort.