Runtime server-side configuration

[enricoros]

Before, during the build phase big-AGI would check for env keys to inform the UI about the backend configuration at that point in time.

Now, there's a single roundtrip between the UI and the backend to re-assess dynamic support for features. This initial framework not only adds env-var validation, but also fixes all the issues experienced with Docker earlier (where the UI kept asking for the keys despite them being on server).

Needs to update docs too.

[enricoros]

Done for now. In the future, we'll adopt mode of backend configuration within the backend, and reflect it in the frontend as well.

Next up shall be some backend-set Auth, which may not even impact the frontend.