Add defaultConfiguration generation option to Config.swift

[fortmarek]

Related: #6248

Short description 📝

Adds an option to specify a default configuration in your Config.swift. We currently use for caching and fetching binaries the first (when alphabetically sorted) debug configuration. As that might not be the desired configuration, we're adding the option to configure the default.

How to test the changes locally 🧐

You can play with the new app_with_custom_default_configuration fixture.

Contributor checklist ✅

• The code has been linted using run mise run lint:fix
• The change is tested via unit testing or acceptance testing, or both
• The title of the PR is formulated in a way that is usable as a changelog entry
• In case the PR introduces changes that affect users, the documentation has been updated

Reviewer checklist ✅

• The code architecture and patterns are consistent with the rest of the codebase
• Reviewer has checked that, if needed, the PR includes the label changelog:added, changelog:fixed, or changelog:changed, and the title is usable as a changelog entry