Improve configuration files structure

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### Details

Proposed changes to improve the configuration structure:
- Split `config.xml` into `packages.xml` (for the packages) and `config.xml` (post installation step inside the tags `apps`, `services`, `path-items`, `registry-items`, and `custom-items`) as it is done in Commando-VM. Note that the installer is already using these two files are we are duplicating the `config.xml` file in our installer. Splitting the configuration will provide a better overview of the configuration and hopefully make it easier for users to configure. It would also allow to configure the packages and keep the default post installations steps. This implies adding an argument to our installer to provide an extra custom file.
- Move all the configuration files (`packages.xml`, and `config.xml`, and `CustomStartLayout.xml`) to a `default-config` folder. This requires modifying the links in the installer. This folder will make it clearer for users which is the default configuration and what can be configured.

@mandiant/flare-vm, @HuskyHacks opinions?

Start by tracing the installer references to config.xml and the duplicated configuration file. Review how packages.xml, config.xml, and CustomStartLayout.xml are currently linked, then verify that moving them into default-config, separating package and post-installation settings, and adding an extra custom-file argument preserves the intended installer behavior.

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