Add feature flag for direct Avro decoder

- Add ReaderProperties::kAvroUseDirectDecoder config flag (default: true)
- Direct decoder is now the default for better performance
- Legacy GenericDatum decoder returns NotImplemented error
- Users can disable direct decoder by setting avro.use-direct-decoder=false
  (will get deprecation error message)

This addresses feedback from wgtmac to add a feature flag as a safety
measure in case of bugs in the new direct decoder implementation.

Author: shangxinli

src/iceberg/avro/avro_reader.cc

@@ -82,6 +82,13 @@ class AvroReader::Impl {
     }
 
     batch_size_ = options.properties->Get(ReaderProperties::kBatchSize);
+    use_direct_decoder_ =
+        options.properties->Get(ReaderProperties::kAvroUseDirectDecoder);
+    if (!use_direct_decoder_) {
+      return NotImplemented(
+          "GenericDatum-based Avro decoder is deprecated. "
+          "Please use the direct decoder (avro.use-direct-decoder=true)");
+    }
     read_schema_ = options.projection;
 
     // Open the input stream and adapt to the avro interface.
@@ -146,7 +153,7 @@ class AvroReader::Impl {
       }
       reader_->decr();
 
-      // Use direct decoder instead of GenericDatum
+      // Use direct decoder for better performance (default path)
       ICEBERG_RETURN_UNEXPECTED(
           DecodeAvroToBuilder(reader_->readerSchema().root(), reader_->decoder(),
                               projection_, *read_schema_, context_->builder_.get()));
@@ -244,6 +251,8 @@ class AvroReader::Impl {
  private:
   // Max number of rows in the record batch to read.
   int64_t batch_size_{};
+  // Whether to use direct decoder (true) or GenericDatum-based decoder (false).
+  bool use_direct_decoder_{true};
   // The end of the split to read and used to terminate the reading.
   std::optional<int64_t> split_end_;
   // The schema to read.

src/iceberg/file_reader.h

@@ -76,6 +76,10 @@ class ReaderProperties : public ConfigBase<ReaderProperties> {
   /// \brief The batch size to read.
   inline static Entry<int64_t> kBatchSize{"read.batch-size", 4096};
 
+  /// \brief Use direct Avro decoder (true) or GenericDatum-based decoder (false).
+  /// Default: true (use direct decoder for better performance).
+  inline static Entry<bool> kAvroUseDirectDecoder{"avro.use-direct-decoder", true};
+
   /// \brief Create a default ReaderProperties instance.
   static std::unique_ptr<ReaderProperties> default_properties();