Without this commit, unsetting any of the `services.kubo.settings` options does not reset the value back to the default. This commit gets rid of this statefulness.
This is achieved by generating the default config, applying the user specified config options to it and then patching the `Identity` and `Pinning` config options from the old config back in. This new config is then applied using `ipfs config replace`.
The only remaining stateful parts of the config are the `Identity` and `Pinning.RemoteServices` settings as those can't be changed with `ipfs config replace`. `Pinning.RemoteServices` also contains secrets that shouldn't be in the Nix store. Setting these options wasn't possible before as it would result in an error when the daemon tried to start. I added some assertions to guard against this case.
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