✨ feat(retention): implement data retention policy

- introduce `Retention` struct to define data retention period and label generation flag
- add `MessageAge` enum to represent different retention time units (days, months, years)
- implement default values for retention policy (5 years, generate label)

src/retention.rs

@@ -0,0 +1,36 @@
+mod message_age;
+
+pub use message_age::MessageAge;
+
+/// Define retention period and flag to indicate if label should be generated
+#[derive(Debug)]
+pub struct Retention {
+    age: MessageAge,
+    generate_label: bool,
+}
+
+impl Default for Retention {
+    fn default() -> Self {
+        Self {
+            age: MessageAge::Years(5),
+            generate_label: true,
+        }
+    }
+}
+
+impl Retention {
+    pub(crate) fn new(age: MessageAge, generate_label: bool) -> Self {
+        Retention {
+            age,
+            generate_label,
+        }
+    }
+
+    pub(crate) fn age(&self) -> &MessageAge {
+        &self.age
+    }
+
+    pub(crate) fn generate_label(&self) -> bool {
+        self.generate_label
+    }
+}