chore: public function generate_unique_snapshot_id for exactly once sink (#42)

* public txn.generate_unique_snapshot_id

* add comment

* empty commit for retry

Author: wcy-fdu

crates/iceberg/src/spec/manifest/_serde.rs

@@ -97,7 +97,7 @@ impl ManifestEntryV1 {
 }
 
 #[serde_as]
-#[derive(Serialize, Deserialize)]
+#[derive(Serialize, Deserialize, Clone)]
 /// Data file
 pub struct DataFileSerde {
     #[serde(default)]

crates/iceberg/src/transaction/mod.rs

@@ -131,7 +131,8 @@ impl<'a> Transaction<'a> {
         Ok(self)
     }
 
-    fn generate_unique_snapshot_id(&self) -> i64 {
+    /// Generate a new `snapshot_id`.
+    pub fn generate_unique_snapshot_id(&self) -> i64 {
         let generate_random_id = || -> i64 {
             let (lhs, rhs) = Uuid::new_v4().as_u64_pair();
             let snapshot_id = (lhs ^ rhs) as i64;