coveo-settings // should support settings as fallback

[jonapich]

You should be able to set another Setting instance as a fallback, and retain the dynamic capabilities.

The first step is to change the typing of the fallback argument to accept Settings instances.

Then, we need to be smart about when we e.g. str(fallback) since it could be NotSet at this point. When the user uses .value to obtain the value we need to ensure we don't trigger the NotSet exception either.