feat(medusa): spread config to have custom props in config

[Inoir]

As i mentioned here: https://github.com/medusajs/medusa/issues/1837 custom props getting stripped of the config. Results in problems in seeding with custom logic and custom props in config, cause there is no way to get this custom props into the service loader i.e.

This allows custom props in the config and still preserves the reserved config from medusa.

Question here is should the custom props inside an object called "customConfig" like @srindom suggested in the issue? currently i dont see the point in changing the config from the real one loaded.

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