feat: Parse markdown in conversation greeting

[s-diez]

Summary

I added support for Markdown in modelSpecs.greeting similar to the implementation of Markdown support in the enviroment variable CUSTOM_FOOTER.

Change Type

Please delete any irrelevant options.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Testing

I run all tests of npm run test:client, all passed.
I then built the container and started the app. With modelSpecs configured as described below. All types of greetings were rendered.

Test Configuration:

modelSpecs:
  list:
    - name: markdown-greeting
      default: false
      label: Markdown Greeting
      description: "Test Markdown"
      preset:
        endpoint: "azureOpenAI"
        model: gpt-4
        modelLabel: Markdown Greeting
        greeting: |
          ## Welcome to Markdown Greeting

          [LibreChat](https://www.librechat.ai/)
    - name: plain-greeting
      default: false
      label: Markdown Greeting
      description: "Test Plain"
      preset:
        endpoint: "azureOpenAI"
        model: gpt-4
        modelLabel: Markdown Greeting
        greeting: |
          Welcome to Plain Greeting
    - name: no-greeting
      default: false
      label: No Greeting
      description: "Test no greeting"
      preset:
        endpoint: "azureOpenAI"
        model: gpt-4
        modelLabel: No Greeting

Checklist

Please delete any irrelevant options.

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have made pertinent documentation changes
• My changes do not introduce new warnings
• Local unit tests pass with my changes
• A pull request for updating the documentation has been submitted.

See LibreChat-AI/librechat.ai#118 for documentation change