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.

Cargo.lock

@@ -540,9 +540,9 @@ dependencies = [
 
 [[package]]
 name = "toml"
-version = "1.1.4+spec-1.1.0"
+version = "1.1.3+spec-1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3aace63f4bbcdfc2c965b059de67119c89c4017a70d633be6c104910f67056f5"
+checksum = "53c96ecdfa941c8fc4fcaed14f99ada8ebed502eef533015095a07e3301d4c3c"
 dependencies = [
  "serde_core",
  "serde_spanned",
@@ -562,9 +562,9 @@ dependencies = [
 
 [[package]]
 name = "toml_parser"
-version = "1.1.3+spec-1.1.0"
+version = "1.1.2+spec-1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1d38ac1cf9b95face32296c0a3ede1fdc270627c9d9c02a7274dd6d960dc4d56"
+checksum = "a2abe9b86193656635d2411dc43050282ca48aa31c2451210f4202550afb7526"
 dependencies = [
  "winnow",
 ]