Add declarative catalog extension for schemas, tables, and views

Extend ReadOnlyCatalogInterface to support declarative definition of
catalog structure using Catalog, Schema, Table, and View descriptor
classes. This provides a more intuitive API for defining catalogs
while maintaining backward compatibility with the existing function-only
pattern.

Key features:
- Catalog/Schema/Table/View descriptor classes in descriptors.py
- Function-backed tables: schema auto-derived from TableFunctionGenerator
- Case-insensitive lookups with original case preserved
- Automatic table_scan_function_get for function-backed tables
- Worker integration propagates catalog attribute to interface

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: rustyconover