Spring-ai Add package org.sf.ai.model that contains abstract base classes for calling AI Models

This package will contains interfaces and abstract base classes that can be used to implement the Client for different data types representing different AI model types. For example text->text or text-messages/text-messages for Conversational/Chat AI Models, text->image for Text to Image generation models.

This will provide a unified 'meta model' across different AI Model types. This will allow contributors to more easily contribute code for AI models that are not yet supported, providing the 'guard rails' or general design of the client that is being contributed.

See #27 for branch with this functionality.