Introduce the UciSource abstraction as a config-rs Source that reads
UCI packages and maps them into the configuration tree. Supports
package selection, required/optional behavior, and custom config/save
directories. Move rust-uci dependency to cfg(unix) to enable UCI
support on all Unix systems for development and testing, while
Linux/OpenWrt remains the production-supported platform.
Ensure the UCI integration is only compiled on Linux, as the underlying native dependency is platform-specific. Added a build script to warn users when the feature is enabled on unsupported platforms.