Metadata integration tests (#57)

* add test for valid metadata v2

* add test for minimal valid table metadata v2

* test valied v1 table metadata

* Test for invalid schema id

* improve failing test

* missing sort oder test

* test for missing partition spec

* test missing partition id

* test missing schemas

* test unsupported version

* fix changes

* improve error message

* fix clippy warnings

crates/iceberg/src/spec/snapshot.rs

@@ -77,7 +77,7 @@ pub struct Snapshot {
     /// A string map that summarizes the snapshot changes, including operation.
     summary: Summary,
     /// ID of the table’s current schema when the snapshot was created.
-    #[builder(setter(strip_option))]
+    #[builder(setter(strip_option), default = "None")]
     schema_id: Option<i64>,
 }