feat(model-controller): Add lookup_field Configuration Option

[eadwinCode]

Overview

Add support for custom lookup fields in ModelController, similar to
Django REST Framework's lookup_field option. This allows using any
unique field (e.g., slug, uuid, key) for object lookups instead of
the default primary key.

Whats Changed:

• Add lookup_field option to ModelConfig (defaults to 'pk')
• Update ModelControllerBuilder to use lookup_field for URL parameters
• Update ModelService.get_one() to support custom lookup fields
• Update endpoint handlers to pass lookup_field to service layer
• Add tests for lookup_field functionality using Client model
• Add documentation with examples (slug, uuid, key fields)