fix: use the custom image factory host in backend and frontend

Contains two fixes:

- In the frontend, `factory.talos.dev` was hardcoded for the maintenance upgrade calls. Move the image factory URL into the `FeaturesConfig` resource, and switch to reading the host from this resource on the frontend.

- In the backend, we were initializing the `imageFactoryHost` in an `init` block, and because of init, it was running before the user override was actually written into the config by Cobra. Fix it by initializing the `imageFactoryHost` lazily when it is used for the first time.

Signed-off-by: Utku Ozdemir <[email protected]>