Add Enhanced Ingestion Mode to genddl Tool

[ron-daniel1]

Adds enhanced ingestion mode to cmd/genddl for generating TPC-DS data
ingestion SQL files.

Key Features:

• Two-stage ingestion: separate source (CSV/TEXTFILE) and target (Parquet) tables
• Catalog support: multi-catalog environments with source/target catalogs
• Format handling: CSV with CAST/NULLIF, TEXTFILE with direct SELECT
• Engine support: Presto (WITH clause) and Spark (USING iceberg + TBLPROPERTIES)
• Correct schema syntax: CREATE SCHEMA catalog.schema WITH (location = 's3a://...')

Files Changed:

New Templates:

• create_source_table.sql.tmpl - Source table DDL (CSV/TEXTFILE)
• create_target_table.sql.tmpl - Target table DDL (Parquet)

Modified: main.go

Schema struct additions:

• Mode string - Detects "enhanced_ingestion" vs legacy mode
• SourceFileFormat string - "CSV" or "TEXTFILE"
• SourceSchema, TargetSchema string - Separate schema names
• SourceCatalog, TargetCatalog string - Multi-catalog support
• S3SourceLocation, S3TargetLocation string - Separate S3 paths
• Engine string - "presto" or "spark" for engine-specific syntax

New functions:

• isEnhancedIngestionMode() - Mode detection helper
• generateSourceTable() - Generates 1a-create-source-*.sql
• generateTargetTable() - Generates 1b-create-target-*.sql

Modified functions:

• loadSchemas() - In enhanced mode, generates only specified catalog type
  (iceberg=true → Iceberg only) instead of all 4 variants
• generateSchemaFromDef() - Routes to enhanced or legacy generation logic
  based on mode
• Run() - Orchestrates enhanced vs legacy workflow

Modified: insert_table.sql.tmpl

• Added conditional CAST/NULLIF for CSV: CAST(NULLIF(column, '') AS type)
• Added direct SELECT * for TEXTFILE format
• Added catalog-qualified table references: source_catalog.source_schema.table

Testing:
✅ All 10 tests pass
✅ Backward compatible - legacy mode unchanged
✅ Generated examples match golden files

Usage:
Enhanced: go run main.go genddl config_enhanced_ingestion.json
Legacy: go run main.go genddl config.json (unchanged)