feat(avro): apply column default values when reading missing fields (3/4)

[huan233usc]

What

Part 3 of 4 of Iceberg v3 column default-value support (POC #731), built on the
schema layer (#746) and the Parquet read path (#792).

When a column is present in the read (table) schema but absent from an Avro data
file — because the column was added after those rows were written — fill it with
the column's v3 initial-default instead of null.

Changes

• arrow/literal_util: add AppendDefaultToBuilder, which appends a
  Literal once to an Arrow ArrayBuilder (for the row-by-row Avro decode
  paths). Like MakeDefaultArray, it targets the storage type for an extension
  builder (arrow.uuid), since Scalar::CastTo has no kernel for extension
  types.
• Avro projection (avro_schema_util.cc): when a field is missing from the
  file and carries an initial-default, project it as
  FieldProjection::Kind::kDefault, mirroring the generic / Parquet paths.
• Avro decode (avro_data_util.cc, avro_direct_decoder.cc): materialize
  the kDefault branch via AppendDefaultToBuilder.

Tests

• literal_util_test: AppendDefaultToBuilder appends a value and casts to the
  builder type.
• avro_data_test: AppendDatumToBuilder fills missing required and optional
  default fields.
• avro_test: end-to-end — write an Avro file with an old schema, then read it
  through ReaderFactoryRegistry with an evolved schema carrying defaults
  (ReadMissingFieldsWithDefaults).

Stack

1. feat(schema): represent, serialize and validate v3 column default values (1/4) #746 — schema: represent / serialize / validate (merged)
2. feat(parquet): apply column default values when reading missing fields (2/4) #792 — read path: Parquet (merged)
3. this PR — read path: Avro
4. schema evolution: addColumn / updateColumnDefault (feat: support v3 column default values in UpdateSchema (3/4) #793)

Draft while the earlier PRs in the stack settle.