feat: add unified cache policy primitives

apps/lib/core/cache/cache_key.dart

@@ -0,0 +1,17 @@
+class CacheKey {
+  final String value;
+
+  const CacheKey(this.value);
+
+  @override
+  String toString() => value;
+
+  @override
+  bool operator ==(Object other) {
+    return identical(this, other) ||
+        (other is CacheKey && other.value == value);
+  }
+
+  @override
+  int get hashCode => value.hashCode;
+}