feat: implement direct Avro encoder for performance

Implement direct Avro encoder to eliminate GenericDatum intermediate layer,
matching the decoder approach for better performance.

Implementation:
- Add avro_direct_encoder_internal.h with EncodeArrowToAvro API
- Add avro_direct_encoder.cc implementing direct Arrow→Avro encoding
  - All primitive types: bool, int, long, float, double, string, binary
  - Temporal types: date, time, timestamp
  - Logical types: uuid, decimal
  - Nested types: struct, list, map (both string and non-string keys)
  - Union type handling for optional fields
- Modify avro_writer.cc to use DataFileWriterBase with direct encoder
- Add EncodeContext to reuse scratch buffers and avoid allocations

This matches Java Iceberg implementation using Encoder interface directly,
avoiding intermediate object allocation overhead.

Author: shangxinli

src/iceberg/CMakeLists.txt

@@ -153,6 +153,7 @@ if(ICEBERG_BUILD_BUNDLE)
       arrow/arrow_fs_file_io.cc
       avro/avro_data_util.cc
       avro/avro_direct_decoder.cc
+      avro/avro_direct_encoder.cc
       avro/avro_reader.cc
       avro/avro_writer.cc
       avro/avro_register.cc