SnapshotRetention::Tag max_ref_age_ms should be optional (#391)

Author: c-thiel

crates/iceberg/src/catalog/mod.rs

@@ -982,7 +982,9 @@ mod tests {
             ref_name: "hank".to_string(),
             reference: SnapshotReference {
                 snapshot_id: 1,
-                retention: SnapshotRetention::Tag { max_ref_age_ms: 1 },
+                retention: SnapshotRetention::Tag {
+                    max_ref_age_ms: Some(1),
+                },
             },
         };
 

crates/iceberg/src/spec/snapshot.rs

@@ -351,7 +351,8 @@ pub enum SnapshotRetention {
     Tag {
         /// For snapshot references except the main branch, a positive number for the max age of the snapshot reference to keep while expiring snapshots.
         /// Defaults to table property history.expire.max-ref-age-ms. The main branch never expires.
-        max_ref_age_ms: i64,
+        #[serde(skip_serializing_if = "Option::is_none")]
+        max_ref_age_ms: Option<i64>,
     },
 }