Add LLM client factory

What does this MR do and why?

Add JH LLM client factory.

It is the factory class for JH AI providers to create clients.

Its default AI provider is chat_glm ( without any config ).

And, allows to configure the default AI provider of the GitLab instance through gitlab.yml.

Yaml format is as follows:

production: &base
  ## GitLab settings
  gitlab:
    ai_provider: chat_glm

Screenshots or screen recordings

Non-UI

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.