feat(monocore): add OCI image reference implementation (#131)

- Enhance Reference struct to handle both tag and digest-based selectors
- Move selector logic from selector.rs into reference.rs
- Improve validation and parsing of OCI image references
- Add comprehensive test coverage for reference parsing and validation
- Remove selector.rs as its functionality is now part of reference.rs

monocore/lib/oci/mod.rs

@@ -7,12 +7,12 @@
 
 mod implementations;
 mod pull;
-mod selector;
+mod reference;
 
 //--------------------------------------------------------------------------------------------------
 // Exports
 //--------------------------------------------------------------------------------------------------
 
 pub use implementations::*;
 pub use pull::*;
-pub use selector::*;
+pub use reference::*;