Spring-ai Remove chain package

The chain idea from langchain involves 'workflow' and often type unsafe access to input and output. Instead of following down that path, first extract the key helper classes that are needed for use cases and later revisit how they can be composed into a chain/flow. There is some example usage of 'chaining' based on langchain's example using java.util.Function in https://github.com/spring-projects-experimental/spring-ai/issues/108

Helper classes envisioned are GenerationTemplate (similar to LLMChain) and ChatEngine (similar to conversationalchain)

Comment From: markpollack

Also remove the 'memory' package

Comment From: markpollack

Remove references in docs.