Add helpers to handle ChatML instruction prompts

[vollnhals]

Description

Provide a new module chatml_utils with two helper functions messages_to_prompt and completion_to_prompt so that ChatML style instructions can be easily implemented.

Fixes #8161

Type of Change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

I am using this in a project of mine with a Mistral LLM.

This change could use some automated tests, but I am not familar with the llama_index repo. Please guide me if you want unit tests.

• Added new unit/integration tests
• I stared at the code and made sure it makes sense

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added Google Colab support for the newly added notebooks.
• My changes generate no new warnings. Don't know?
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I ran make format; make lint to appease the lint gods