Decouple system config from user overrides in ipfs/conf

[schomatis]

Toward #8925.

Still a WIP (there are still decisions that need to be made and broken tests to be fixed), but it has enough content to start the review process.

Some notes (will flesh this out later):

• A new type UserConfigOverrides is introduced in the middle of the coupling of system Config and user JSON .ipfs/conf file.
• The PR was written around the Repo interface, reworking it to use this new type where possible.
• The profiles, before implicit in the serialized Config is now explicit in the user config file.
• There are pending decisions around the config UX having in mind reducing the impact on the user. In general we default to behaving as much as possible as we have been doing.