refactor: simplify NestedField constructors (#1136)

Signed-off-by: xxchan <[email protected]>## Which issue does this PR
close?

- Closes #.

## What changes are included in this PR?



## Are these changes tested?


Signed-off-by: xxchan <[email protected]>
Co-authored-by: Renjie Liu <[email protected]>

Author: xxchan

crates/iceberg/src/spec/datatypes.rs

@@ -608,37 +608,17 @@ impl NestedField {
 
     /// Construct a required field.
     pub fn required(id: i32, name: impl ToString, field_type: Type) -> Self {
-        Self {
-            id,
-            name: name.to_string(),
-            required: true,
-            field_type: Box::new(field_type),
-            doc: None,
-            initial_default: None,
-            write_default: None,
-        }
+        Self::new(id, name, field_type, true)
     }
 
     /// Construct an optional field.
     pub fn optional(id: i32, name: impl ToString, field_type: Type) -> Self {
-        Self {
-            id,
-            name: name.to_string(),
-            required: false,
-            field_type: Box::new(field_type),
-            doc: None,
-            initial_default: None,
-            write_default: None,
-        }
+        Self::new(id, name, field_type, false)
     }
 
     /// Construct list type's element field.
     pub fn list_element(id: i32, field_type: Type, required: bool) -> Self {
-        if required {
-            Self::required(id, LIST_FIELD_NAME, field_type)
-        } else {
-            Self::optional(id, LIST_FIELD_NAME, field_type)
-        }
+        Self::new(id, LIST_FIELD_NAME, field_type, required)
     }
 
     /// Construct map type's key field.
@@ -648,11 +628,7 @@ impl NestedField {
 
     /// Construct map type's value field.
     pub fn map_value_element(id: i32, field_type: Type, required: bool) -> Self {
-        if required {
-            Self::required(id, MAP_VALUE_FIELD_NAME, field_type)
-        } else {
-            Self::optional(id, MAP_VALUE_FIELD_NAME, field_type)
-        }
+        Self::new(id, MAP_VALUE_FIELD_NAME, field_type, required)
     }
 
     /// Set the field's doc.