[presto][iceberg] Add Variant type support for Iceberg V3 (#27374)

[apurva-meta]

Summary:

Add support for Iceberg V3 VARIANT type across the Presto-Iceberg connector type
conversion pipeline, including a binary codec for the Apache Variant spec and
SQL scalar functions for Variant data manipulation.

The VARIANT type represents semi-structured data (JSON) and is mapped to Presto's
unbounded VARCHAR type. This diff provides:

1. Type mapping: VARIANT to/from VARCHAR across all converter layers
2. VariantBinaryCodec: Full encoder/decoder for Apache Variant binary format
   • Supports primitives (null, bool, int8/16/32/64, double, string)
   • Supports short strings (0-63 bytes) and long strings
   • Supports objects (with metadata dictionary) and arrays
   • JSON string round-trip: JSON -> binary -> JSON
   • Binary format detection (isVariantBinary) and auto-decode (decodeVariantAuto)
   • Type introspection from binary headers (getValueTypeName)
3. SQL scalar functions registered via IcebergConnector.getSystemFunctions():
   • variant_get(varchar, varchar): Extract field with dot-path and array indexing
     (e.g., 'users[0].name', 'address.city')
   • variant_keys(varchar): Return top-level object keys as JSON array
   • variant_type(varchar): Return JSON type name (object, array, string, number, boolean, null)
   • to_variant(varchar): Validate JSON and cast to Variant (Phase 5 CAST support)
   • parse_variant(varchar): Validate and normalize through Variant binary codec
   • variant_to_json(varchar): Normalize Variant to compact JSON representation
   • variant_binary_roundtrip(varchar): Encode to binary and decode back (interop testing)
4. Predicate pushdown: IS NULL/IS NOT NULL works through VARCHAR mapping;
   variant_get pushdown tracked as future optimizer rule work
5. Comprehensive tests for the codec, functions, and end-to-end connector behavior

Changes:

• TypeConverter: Map VARIANT to VarcharType (unbounded) in toPrestoType(), and to
  ORC STRING type in toOrcType()
• IcebergUtil: Handle VARIANT partition values as string slices in domain creation
• PartitionData: Deserialize VARIANT partition values as text (same as STRING)
• PartitionTable: Convert VariantType partition values to string representation
• VariantBinaryCodec: Full Apache Variant binary spec (v1) encoder/decoder with
  binary detection, type introspection, and auto-decode capabilities
• VariantFunctions: 7 SQL scalar functions for Variant data manipulation including
  dot-path navigation, array indexing, key enumeration, type introspection, CAST
• IcebergConnector: Register VariantFunctions in getSystemFunctions()
• TestVariantBinaryCodec: 40+ tests covering primitives, objects, arrays, metadata,
  binary detection, type names, and auto-decode
• TestVariantFunctions: 40+ tests covering all 7 functions including dot-path,
  array indexing, error handling, and edge cases
• TestIcebergV3: Integration tests for VARIANT type including JSON data round-trip

Note: Velox/Prestissimo does not require changes - the VARIANT->VARCHAR type
mapping flows automatically through HiveTypeParser on the C++ side.

Differential Revision: D96755027

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