[Tracking] Support LIST and composite types (storage + runtime)

[longbinlai]

Summary

Bring LIST and other composite/nested types (tuple / struct) to first-class support across storage and runtime / ingestion, so that users can model and query nested data without workarounds.

Motivation

Today, support for nested types is fragmented:

• Storage: LIST is partially modeled but lacks recursive component_type=list support; tuple / struct have no dedicated columnar layout (today they would have to be encoded inline, like strings, instead of being split into per-field columns).
• Ingestion (load_data): behavior diverges per format — Parquet has the best LIST handling, JSON partially works, CSV essentially does not. Users hit format-specific cliffs when bringing nested data into the graph.

A unified umbrella lets us land the storage primitives once and have ingestion (and later, query/expression) consume them consistently.

Scope (in)

• Storage layer
  • LIST type with recursive component_type (LIST of LIST, etc.)
  • tuple / struct columnar layout (per-field columns; nested columns)
• Runtime / ingestion layer
  • load_data for CSV / JSON / Parquet — close the format gaps for LIST
  • Round-trip via the columnar storage primitives above

Scope (out)

• New external file formats themselves (tracked under [Feature] External Data Support: Multi-Format, Remote Filesystem, Data Lake, and Optimization #158 External Data Support)
• CAST / temporal type inference (tracked under [Tracking] Improve & Extend COPY FROM: schema, typing, performance, options #221 COPY FROM capability)

Child issues (tracked here)

Storage primitives

• Introduce support for List type in storage. #159 — Introduce support for List type in storage (with recursive component_type)
• Support tuple/struct in columar format #444 — Support tuple / struct in columnar format (per-field columns; nested columns)

Ingestion (load_data) — moved from #221

• CSV: Array (LIST) type not supported in load_data #441 — CSV: Array (LIST) type not supported in load_data
• JSON: Array (LIST) type not fully supported in load_data #442 — JSON: Array (LIST) type not fully supported in load_data
• Parquet: Array (LIST) type not fully supported in load_data #443 — Parquet: Array (LIST) type not fully supported in load_data