I need to be able to read the config at module load time, which makes
async difficult if not impossible.
This will make read config operations synchronous, which is fine. The
file is tiny and seldom read.
This adds a config file that's loaded very early on during startup.
It enables us to save/load settings from within Grist's admin panel, that affect the startup of the FlexServer.
The config file loading:
- Is type-safe,
- Validates the config file on startup
- Provides a path to upgrade to future versions.
It should be extensible from other versions of Grist (such as desktop), by overriding `getGlobalConfig` in stubs.
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Some minor refactors needed to occur to make this possible. This includes:
- Extracting config loading into its own module (out of FlexServer).
- Cleaning up the `loadConfig` function in FlexServer into `loadLoginSystem` (which is what its main purpose was before).
