fix: allow missing sequence-number in v2 snapshots for v1-upgraded tables

Author: jembishop

crates/iceberg/src/spec/snapshot.rs

@@ -293,6 +293,7 @@ pub(super) mod _serde {
         pub snapshot_id: i64,
         #[serde(skip_serializing_if = "Option::is_none")]
         pub parent_snapshot_id: Option<i64>,
+        #[serde(default)]
         pub sequence_number: i64,
         pub timestamp_ms: i64,
         pub manifest_list: String,