Implement endpoint switch to custom

[reechamishra]

Motivation

Implement capability to switch endpoint to custom or byod based on env config to give flexibility to users for their needs.

How would you feel if this feature request was implemented?

Amazing

Requirements

As a chat user I want to switch between "On Your Data" endpoint to a "Custom" endpoint through configuration so that I can understand how to customise the solution for my own needs

Tasks

To be filled in by the engineer picking up the issue

• [x] Introduce a new env variable to choose between custom or byod conversation type.
• [x] Use that new variable in create_app.py and create a new REST binding.
• [ ] Make changes to frontend to honour the new binding.