Add InMemoryCatalog example implementation

Create an in-memory catalog implementation for testing and as a reference:

- InMemoryCatalog class implementing CatalogInterface
- In-memory storage using Python dictionaries for catalogs, schemas, tables, views
- UUID-based attach_id generation
- Supports DDL operations: catalog/schema/table/view create/drop/rename
- Does not support transactions (returns None)
- Catalog versioning for change tracking

Also adds:
- InMemoryCatalogWorker class with catalog_interface set
- Entry point: vgi-example-catalog-worker

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

Author: rustyconover