chore: expose field id mapping from schema (apache#1498)

## What changes are included in this PR?

Motivation:
- I would like to leverage naming mapping, to compensate the issue that
table metadata reassigns field id on creation.
- Name mapping requires (1) field id; (2) field name; (3) children
values; all these information could be fetched from `Schema`, but could
be a lot easier and performant if we could expose (1) field id -> field
name; (2) field id -> field mapping.

## Are these changes tested?

N/A, trivial visibility change, no functional change

Author: dentiny

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

@@ -410,9 +410,14 @@ impl Schema {
     }
 
     /// Return A HashMap matching field ids to field names.
-    pub(crate) fn field_id_to_name_map(&self) -> &HashMap<i32, String> {
+    pub fn field_id_to_name_map(&self) -> &HashMap<i32, String> {
         &self.id_to_name
     }
+
+    /// Return a hashmap matching field ids to nested fields.
+    pub fn field_id_to_fields(&self) -> &HashMap<i32, NestedFieldRef> {
+        &self.id_to_field
+    }
 }
 
 impl Display for Schema {