Generalize configProvider

[timroes]

Currently the config provider in the FE does mix it's configuration from build time environment variables (REACT_APP_), variables assigned to window and a file loaded from config.json.

We want to narrow this down to two different providers with different priority, i.e. window or config.json should exist and be able to overwrite the build time environment variables. The code should be generic to be able to overwrite any part of the configuration without requiring hard-coded references for each individual variable.

[octavia-squidington-iii]

cc @airbytehq/frontend

[timroes]

After some brainstorming with Teal, Topher and Davin, we decided to change how we're handling environment variables. In the future we only want to except all environment variables via build time environment variables (i.e. REACT_APP_...). We'd instead build the code for each staging/dev environment separately (we anyway do that for the docker images). Therefore we'll remove any configuration via the window object or (the not yet enabled) config.json file. This will significantly decrease the complexity of our configProviders code and offload the efforts to the build time.

This will require #15648 to be solved, since the demo instance at the moment still requires to set the IS_DEMO variable on window.