chore: expose default schema id (#1526)

## What changes are included in this PR?

I'm using `SchemaBuilder` to construct `Schema`, I want to do assertion
on schema id to make sure it's properly assigned.
Essentially:
```sh
// upstream
let schema = SchemaBuilder::new().xxx().xxx().build();

// downstream
assert_ne!(schema.schema_id(), DEFAULT_SCHEMA_ID);
```

## Are these changes tested?

No-op change, compilation is enough.

Author: dentiny

crates/iceberg/src/spec/schema/mod.rs

@@ -49,7 +49,8 @@ use crate::{Error, ErrorKind, ensure_data_valid};
 pub type SchemaId = i32;
 /// Reference to [`Schema`].
 pub type SchemaRef = Arc<Schema>;
-pub(crate) const DEFAULT_SCHEMA_ID: SchemaId = 0;
+/// Default schema id.
+pub const DEFAULT_SCHEMA_ID: SchemaId = 0;
 
 /// Defines schema in iceberg.
 #[derive(Debug, Serialize, Deserialize, Clone)]