Add default use case selection based on `io.element.default_use_case` .well-known key

[Half-Shot]

Implementation of https://github.com/vector-im/element-meta/issues/525

Checklist

• [ ] Tests written for new code (and old code if feasible)
• [ ] Linter and other CI checks pass
• [ ] Sign-off given on the changes (see CONTRIBUTING.md)

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