feat: eliminate GenericDatum in Avro reader for performance (#374)

Replace GenericDatum intermediate layer with direct Avro decoder access
to improve manifest I/O performance.

Changes:
- Add avro_direct_decoder_internal.h with DecodeAvroToBuilder API
- Add avro_direct_decoder.cc implementing direct Avro→Arrow decoding
- Primitive types: bool, int, long, float, double, string, binary, fixed
  - Temporal types: date, time, timestamp
  - Logical types: uuid, decimal
  - Nested types: struct, list, map
  - Union type handling for nullable fields
- Modify avro_reader.cc to use DataFileReaderBase with direct decoder
  - Replace DataFileReader<GenericDatum> with DataFileReaderBase
  - Use decoder.decodeInt(), decodeLong(), etc. directly
  - Remove GenericDatum allocation and extraction overhead
- Update CMakeLists.txt to include new decoder source

Performance improvement:
- Before: Avro binary → GenericDatum → Extract → Arrow (3 steps)
- After: Avro binary → decoder.decodeInt() → Arrow (2 steps)

Author: shangxinli

src/iceberg/CMakeLists.txt

@@ -148,6 +148,7 @@ if(ICEBERG_BUILD_BUNDLE)
   set(ICEBERG_BUNDLE_SOURCES
       arrow/arrow_fs_file_io.cc
       avro/avro_data_util.cc
+      avro/avro_direct_decoder.cc
       avro/avro_reader.cc
       avro/avro_writer.cc
       avro/avro_register.cc

src/iceberg/avro/CMakeLists.txt

@@ -16,3 +16,9 @@
 # under the License.
 
 iceberg_install_all_headers(iceberg/avro)
+
+# avro_scan benchmark executable
+add_executable(avro_scan avro_scan.cc)
+target_link_libraries(avro_scan PRIVATE iceberg_bundle_static)
+set_target_properties(avro_scan PROPERTIES RUNTIME_OUTPUT_DIRECTORY
+                                           "${CMAKE_BINARY_DIR}/src/iceberg/avro")