[`mgt-chat`] Ability to toggle model B for API calls

[sebastienlevert]

The chat component needs a new property usePremiumApis that is a boolean and that automatically appends model=B to all calls that support premium APIs.

We should be able to test with a real monetized app. We should render an error message if the conditions are not met (app is not a monetized app)

[sebastienlevert]

yejuntak to design a specific error message for this.

[yejuntak]

Sébastien Levert (@sebastienlevert) Gavin Barron (@gavinbarron) Should the error message be a pop up or render inside of chatbox?

[sebastienlevert]

Same as the other error messages, within the chat thread. Also we probably should disable the sendbox.