feat(uci): add generalized UCI configuration source

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.

src/platform/unix/mod.rs

@@ -0,0 +1,2 @@
+#[cfg(feature = "uci")]
+pub mod uci;