[FEATURE] Create tutorial for connector process function

[ylwu-amzn]

Add tutorial docs for how to build pre/post process functions for connector.

[ylwu-amzn]

preprocess function can only support embedding , can we extend it?

[ramda1234786]

@ylwu-amzn , will this support LLM of HF like mentioned here in the feature request. Currently only broken piece is making post_process_request else i am able to predict any Hugging Face Models

https://github.com/opensearch-project/ml-commons/issues/1869