Add decimal builders to RBB

Author: ctsk

arrow-array/src/builder/mod.rs

@@ -514,8 +514,8 @@ pub fn new_empty_builder(data_type: &DataType, capacity: usize) -> Box<dyn Array
         DataType::Struct(_fields) => todo!(),
         DataType::Union(_union_fields, _union_mode) => todo!(),
         DataType::Dictionary(_data_type, _data_type1) => todo!(),
-        DataType::Decimal128(_, _) => todo!(),
-        DataType::Decimal256(_, _) => todo!(),
+        DataType::Decimal128(_, _) => Box::new(Decimal128Builder::with_capacity(capacity)) as _,
+        DataType::Decimal256(_, _) => Box::new(Decimal256Builder::with_capacity(capacity)) as _,
         DataType::Map(_field, _) => todo!(),
         DataType::RunEndEncoded(_field, _field1) => todo!(),
         dt => panic!("Unexpected data type {dt:?}"),