Adds the `networking.networkmanager.connectionConfig` option which allows setting arbitrary settings inside the `[connection]` section.
This also reworked the underlying representation significantly to be less string-pasting and more semantic. In a future step it probably makes sense to provide raw access to other sections to users rather than replying on `extraConfig`. However I decided to defer this primarily because ordering of sections can matter. (Although IIUC this is only true for different `[connection]` sections). I think in the future we could expose an object where users can define/edit all sections and map the current configuration onto those. For now however only `[connection]` is exposed and the rest are just used internally.
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