Add STRUCT/ARRAY/MAP nested column support to RAB (CSA-371)

[bladata1990]

Summary

• Adds support for complex SQL types (STRUCT, ARRAY, MAP<K,STRUCT>) in the Relational Assets Builder package
• Child fields of complex-typed columns are emitted as separate Column assets linked via parentColumn hierarchy
• Sub-columns are correctly excluded from the table's flat Columns list — they only appear inside the parent column's nested view

Key design decisions

Why sub-columns don't appear in the flat Columns list:
Atlan auto-creates the table_columns relationship from tableQualifiedName server-side. Sub-columns have all table/view reference fields cleared (tableQualifiedName, tableName, table, viewQualifiedName, viewName, view, materializedView) so they are NOT added to table_columns. Navigation is via the parentColumn chain.

QN format (matching Databricks connector):

• STRUCT field: tableQN/parentCol/fieldName
• ARRAY: tableQN/parentCol/items/fieldName
• MAP<K,STRUCT>: tableQN/parentCol/values/fieldName
• Deeply nested: tableQN/parentCol/outerField/innerField

Fields set on sub-columns:

• parentColumnQualifiedName / parentColumn / parentColumnName
• columnHierarchy: newline-delimited JSON ancestor entries (enables breadcrumb display)
• columnDepthLevel: 1 for direct fields, 2+ for deeper nesting
• nestedColumnOrder: ordinal within parent
• subType=nested

Files changed

• ComplexTypeParser.kt — NEW: bracket-aware recursive parser for STRUCT/ARRAY/MAP type strings
• AssetXformer.kt — added nested column fields to BASE_OUTPUT_HEADERS
• ColumnXformer.kt — overrides mapRow() to recursively emit child column rows for complex types
• ComplexTypeParserTest.kt — NEW: unit tests for the parser
• assets-complex.csv — NEW: test fixture with STRUCT/ARRAY/MAP types
• build.gradle.kts (RAB + AIM) — minor build fixes

Test plan

• Tested on fs3.atlan.com with Redshift connector (ci_noqn_test table)
• plain_col, struct_col, nested_struct_col appear in table's flat Columns list (3 total)
• city, zip appear only inside struct_col's nested view
• outer, label appear only inside nested_struct_col's nested view
• inner, count appear only inside outer's nested view (depth=2)
• columnHierarchy breadcrumbs display correctly in the UI
• Run unit tests: ./gradlew :samples:packages:relational-assets-builder:test -PpackageTests

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