feat: add support for apache iceberg through duckdb iceberg extension

[dheerapat]

Tested using Cloudflare R2 REST catalog

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Summary by cubic

Add Apache Iceberg support using the duckdb Iceberg extension and a REST catalog. You can now run LEA against Iceberg tables (tested with Cloudflare R2).

• New Features

  • Added Warehouse.ICEBERG with IcebergClient built on duckdb’s Iceberg extension (ATTACH as iceberg_catalog).
  • Supports token or OAuth2 client credentials for REST catalogs.
  • Table ops: create-as-select, clone, delete-and-insert, and delete; lists tables, fields, and stats.
  • Serialized execution for Iceberg (max_workers=1) and catalog selection via USE iceberg_catalog.
  • Note: header statements are not supported for Iceberg scripts.

• Migration

  • Set env vars:
    • LEA_WAREHOUSE=iceberg
    • LEA_ICEBERG_WAREHOUSE, LEA_ICEBERG_ENDPOINT
    • Auth: LEA_ICEBERG_TOKEN or LEA_ICEBERG_CLIENT_ID/LEA_ICEBERG_CLIENT_SECRET (optional LEA_ICEBERG_OAUTH2_SERVER_URI)
  • The duckdb Iceberg extension is installed and loaded automatically.

^{Written for commit ad75ba7. Summary will update on new commits.}