* feat: implement global cache usage in rules
* refactor: make global cache generic with local key types
- Remove GlobalKey struct from gosec_cache.go
- Each use case now defines its own key type (type safety via Go's type system)
- Move RegexMatchWithCache to separate regex_cache.go file
- Move cache kind constants to rules/hardcoded_credentials.go as local types
- Add documentation for cache key requirements
